It is satisfying to work when I receive usable feedback; so I particularly appreciate situations like this one. You don't have to change the lifters on both sides. If you use Sealed Power lifters, it should not make a noticeable difference in performance (except for hopefully stopping the noise). This year motor uses roller lifters. They are sensitive to the location of the oil hole when they are installed; so you need to be careful to match the orientation of the new lifters to that of the old ones. It is also recommended to submerge new lifters in clean motor oil for some time before installation; in order to pre-fill them with oil and to lubricate the cam lobes that contact them.
Sometimes old lifters will be difficult to remove. They should be able to be pulled straight out; after the push rods are removed. But you might need a well made pair of pliers to grasp them or work them around to break the deposits. There are specialized tools available for pulling lifters out of their bores. Snap On and Matco tool distributors would be sources for those types of tools. NAPA parts stores may also have or be able to order them. You may or may not need such luxuries.
The time required for replacing the lifters could vary greatly; depending on how difficult it is to pull them out. But other than that; it should not be significantly longer than the time required to replace the pushrods (assuming that you pulled the intake manifold to replace the pushrods). If you didn't pull the manifold to replace the pushrods; then it will be a few hours longer.
Incidentally; black smoke is not indicative of engines with worn pistons. Black smoke is caused by an excessively rich fuel mixture; which has nothing to do with pistons (except that engines which have deteriorated to the point where the pistons are worn often also have fuel system problems). Worn pistons will produce blue smoke. But piston slap is frequently not associated with any smoke at all.
Piston slap is caused by the sides of the piston skirts wearing to the point where the pistons can rock a little from side to side on the wrist pin. This can happen if the piston skirt loses just a few thousandths of an inch in diameter. It is annoying, but is not necessarily a problem that requires repair. My 318 now has TRW forged pistons in it. Forged pistons are typically made with greater skirt to cylinder wall clearance than cast pistons. So my pistons will sometimes slap while the motor is warming up; but the noise goes away after the motor gets up to full temperature. I have also found that the piston slap in my engine is very much affected by the formulation and weight of motor oil being used. I broke this motor in on Mobil 1; in 0W-30 weight, (which was the first super wide viscosity full synthetic multigrade they made, in 1997). But the pistons were noisier than I liked. I later changed to Mobil 1 in 0W-40; which was the only type of Mobil 1 that was available in New Mexico when I stopped there to add oil on a cross country trip. And I was amazed at how much quieter the engine became; and how much less oil it now uses. So I have become totally sold on this viscosity (which is a European non-energy conserving formula). But like I said (or I hope I said) before; piston slap cannot be heard at idle. It only becomes audible after the engine speed increases a little beyond idle. If your engine knocks at idle; it is NOT piston slap. However; it could be a broken piston skirt or a loose wrist pin (which was a not uncommon issue with Chrysler motors of the past).
BTW, I would NOT recommend going to synthetic oil in an old motor like yours. But it would be an excellent choice to go to 20W-50 petroleum oil in an old Chrysler motor. I would recommend Pennzoil, Castrol GTX, Texaco Havoline or Kendall GT-1, in that viscosity. Good oil filtration is also a big advantage in a sludgy motor. I would use a Fram Tough Guard # TG16; or a # TG8A if there is enough room for a longer filter in that vehicle.
It is also possible that the noise comes from valves which are hanging up in their guides. This would not be surprising on a motor which has not been run for some time. So here's what I would do first: buy a large bottle of Chevron Techron fuel system and combustion chamber cleaner (at Chevron gas stations, Wal Mart, Auto Zone, or Checker, Shucks, Kragen, Murray, or O'Reilly parts stores). Add the full bottle to the fuel tank at a filling station and then fill the tank with fuel (or if the cost is too high; at least add enough fuel so you're sure there is 15 or more gallons in the tank). Drive the Jeep for 50 to 75 miles; in order for this miracle product to remove the deposits from the combustion chambers and valves. And see how much difference this treatment makes. It might make you decide to not tear into the motor again. Also, when a 318 has been stored for some time; it might require driving some distance before the air bleeds out of the lifters and they quiet down.
If the noise does not go away, and you decide to replace the lifters; I strongly recommend first flushing the lubrication system to minimize the quantity of sludge that will end up in the new lifters. There are many products on the market for flushing engines; but I only recommend using an unusually effective combination that I have worked out over the years. It involves a bit of effort; but it is well worth it.
Order a quart (or more) of Kreen internal engine cleaner and lubricant from (www.kanolabs.com). They have a special on their website for that item. Also buy locally a quart of Pennzoil "ATF" automatic transmission fluid. And buy one of the two Fram Tough Guard oil filter models I recommended and 6 quarts of one of the oils I recommended above.
When the Kreen arrives; you'll need to have the oil level in the Jeep at least a quart low. If it is not low; loosen the oil drain plug, and drain out enough oil to drop the level to at least a quart low. Then mix 1/2 quart of Kreen with 1/2 quart of Pennzoil ATF, and add this mixture to the crankcase. If the oil level is then more than 1/2 quart low, add enough of the oil you just bought to bring it up to full. Also add 1/2 quart of Kreen to the fuel tank.
Drive the car gently for 1/2 to 3/4 hour; keeping the speed below 50 mph, and not going up steep hills; then come back and drain the oil while the engine is still warm. Replace the oil filter, and fill the crankcase with new oil. Drive the car and then decide whather you still want to replace the lifters.