1) What was your diagnostic process that narrowed it down to a tappet? I'm only asking because, if you have the 6-242, swapping a tappet is not an easy job. You see, the engine needs to be decapitated...
2) It's not necessary to go with OEM parts, I rarely do. I've been meaning to try Rhoads tappets for the 6-242 next time I need a set and I'm not in a hurry - I've used them on V8 buildups with great results over the years. However, any qualify aftermarket part (I used to use a lot of TRW) will serve handily. Just like engine bearings - Clevite77 when I can find them, TRW when I couldn't, OEM was a weapon of last resort (going OEM doesn't make much of a difference in anything but your wallet, by the time you add it all up.)
3) It is correct that production camshafts are ground more to meet emissions than to help fuel economy or efficiency (differentiation later - they are different.) That's also why so many aftermarket bumpsticks have the caveat of "not for use on Emissions-Controlled motor vehicles.)" However, it should be noted that they often will pass emissions testing, it just can't be guaranteed and the manufacturer doesn't want to assume the liability. There are many reasons why a particular grind would not pass emissions. (Which is irritating. If California would get out of my way and let me do things right, I could eliminate nearly everything except the CCV setup and make the engine run cleaner thereby - including benefitting both fuel economy and fuel efficiency!)
Swapping a cam is a fair amount of work - not only will you need to pull the tappets out (it can be done without removing the head, but it takes effort and patience, and creativity to hold the tappets up out of the cam bore...) you will also need to disassemble the front of the engine and dismount the radiator (the AMC 6 camshaft is about two feet long, and iron doesn't bend very well. You Have Been Warned.) Therefore, swapping the cam is probably not something I'd do "just on account of 'cause" (you can, but it's a headache,) I'd rather do it as a core element of (and generally in concert with elements of) a comprehensive performance plan. Then again, that's also usually done on the engine stand, not in the vehicle.
However, altering valve timing events (via a camshaft swap) is a viable method for gaining torque numbers and potentially moving the torque peak down through the RPM band - which can help significantly, depending on intended use! Playing with cam timing (within limits, natch) also has a beneficial planned effect on performance peak locations - I cover this in the camshaft chapter of The Jeep I6 Power Manual (check my site for info.) The only problem there is that cam indexing parts can be difficult to get, about all that can be available would be offset crankshaft sprocket keys that happen to fit other applications.
It really depends on what you want to do with the truck - but the biggest "bang-for-buck" driveway-doable mod is to take the crankshaft and connecting rods from the AMC 6-258 (4.2L) and install them - this kicks your total displacement up to around 270-280ci, and has a fairly major effect on low-end torque as well - more than the typical camshaft swap! (I cover this in the Strokers chapter in the Power Manual as well, giving examples of the various changes and changes in power output.)
Believe me, there's nothing at all wrong with wanting more torque from your truck's engine! However, just shotgunning parts at it often has negatively synergistic results; while if you plan it all out beforehand, you can usually note significant gains and dream out a plan you can either do all at once or do piecewise - depending on what you're trying to do. Anticipated use is very much a factor...
EDIT - forgot...
OK, so I didn't retain what I read in your OP. You have the 6-242. Rock solid engine, good performance basis, just a limited aftermarket. Workable (and preferable!)
"Fuel economy" - the unit quantity fuel burned per mile travelled, typically expressed in reverse as "miles per gallon." This is not the same thing as fuel efficiency, but it is a key factor in most vehicle purchases (don't confuse fuel economy with power - while the two are not strictly mutually exclusive, production vehicles in general take an "either/or" approach to the matter.)
"Fuel efficiency" - the unit power/torque produced per unit mass of fuel burned. High fuel efficiency and high fuel economy can be (and often are) coupled, as having the high unit power available can make it easier to travel per mile, and therefore reduce the unit mass of fuel required per mile.
Efficiency is simple to maximise on any vehicle - cruise at or near the peak torque crankshaft speed of the engine. When I stopped using fifth gear in my 88, I gained three MPG or so! Why? Because the way the vehicle is geared with the manual transmission and the 6-242 (3.07:1,) the engine ran entirely too slowly in overdrive at freeway speed with 30-31" tyres. By using direct drive (fourth gear) instead of overdrive (fifth,) I moved my cruising crankshaft speed from ~1900-2100rpm to ~2600-2800rpm, bringing it more in line with the torque peak output of the engine. Any piston-engine aircraft pilot or ship's engineer will tell you the same thing - to maximise cruise range, cruise at peak torque output of the engine. That's when the engine moves air the most efficiently (volumetric efficiency is highest,) which allows it to not only burn more fuel, but do so more efficiently. Fuel injection systems are responsive to engine loading conditions, so it knows to reduce fuel as necessary to improve economy at peak efficiency, if the production programmers and tuners paid attention to what they were doing...
"recon" (sic - reckon)(tm) "hihgly"(tm) "seceed"(tm)
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