Q: Camshaft interchangeability 4.2 & 4.0

No, you're not coming across as rude at all! I don't want to be doing this all over again in 6 months, so I'd much rather get bad news or be told I've got something totally wrong now than discover it the hard way when I last expect it. To answer your question:

The odometer reads 107k and change, but I wouldn't be surprised if there were many more miles on it, based on how tired some of the wear components are. While I didn't find metal in the pan, I had originally thought I'd identified a crack in the crankshaft, which is what prompted me to do a full teardown and inspection of the engine, top to bottom. When doing this, I found that the aftermarket HEI distributor I was using had a hardened gear (despite assurances that this wasn't the case; live and learn, I guess), evidenced by the camshaft's drive gear having been eaten away to near nothing. It might have had 100 miles left in it before it would've been wiped out, if I had to guess based on amount of wear and number of miles I had been running it for. 

On top of that, the timing chain was sloppy as all get out, so I'm putting in a new set (thinking double roller is the way to go, based on the short lifespans of OE spec replacements people have been mentioning on the internet for the past 10 years or so). Harmonic balancer rubber was cracked, uneven, and squeezing out in places, already got a replacement for that. All the lifters (except maybe one or two) were totally stuck. Pretty much solid, really. There was some indication of very light scuffing on the valve stems (undetectable by touch, but a visible difference, like between a gloss and satin finish). Pushrods were fine — not even clogged or anything. Just had some varnish on em. They're still getting replaced, 4.0 cam or no.

That's a whole lot of words to say, even though I didn't find metal in the pan, it was clearly all throughout the engine (unless the cam fairy decided to magic away all those gear fragments as they were getting torn off haha). So because of all this, everything's been pulled apart, all the bearings are getting replaced — EDIT: including the cam bearings, unfortunately. I failed to use that stuff between my ears and didn't think about em when I soaked the block in degreaser, so that lovely golden/brassy bearing coating all got dissolved away. Very embarrassing and time consuming mistake to make. 

For the record, to anyone who comes across this in the future: LA's Totally Awesome is the best and cheapest degreaser I've ever used, but it will eat aluminum, zinc, tin, babbitt, and most other soft, non-ferrous metals commonly found in machine parts. Be more careful than I was!

I intend to reuse the following:

• Pistons (pending bore & clearance measurements, which is what I'm working on right now), wrist pins, rods, and crank

• Springs, retainers, rockers, pivots, bridges

• Oil pump (but it's getting a full rebuild if it passes inspection once I get to that)

And this is all on top of the tranny rebuild I was doing that kicked all this off haha!

I think I should probably look more into determining whether the rockers and pivots should or can be reused. They look okay at a glance, but I don't want to leave that to little more than a guess. I'm kinda iffy on the springs too, mostly because I don't have a testing tool and haven't yet had a chance to look into what options I might have to reliably test them. 

Usually I'd just replace the rockers, springs, whole shebang along with everything else and not give it a second thought, but all the engine work I'm doing was totally unplanned and I'm of fairly limited resources at the moment. Gotta find ways to cut expenses. At the same time, it's that "I'm too poor to buy cheap tools" thing, so I get that I won't be saving a dime if penny pinching causes the cash and work put into this to go up in smoke. 

So far it's looking as though there's not going to be a need for any machine work to be done. Cylinders are all in range for taper and tolerance, but I'm gonna be extra sure about that and have my buddy come out and do a sanity check to make sure my desire isn't throwing off my ability to measure correctly. You make a good point about at least checking the price of a short block. It's not just the expense of machine work around here, but the lead time, too. Just about everybody seems to be backed up for weeks at present, which I'm not gonna begrudge them over if that means they get to keep the lights on!