Do you need to align bore the pumpkin when replacing the cap?
The reason I'm asking is I have an opportunity to buy a chunk for a good price, but it comes with 4 loose caps. How critical is the match?

With the 8 3/4 you have two issues with the caps, first they must hold the races tight and the adjusters must spin so you can adjust the backlash and preload. If you line bore them you have to tap the threads as well, because the boring will remove some of the threads. At least in my area no machine shop can handle that type of repair at least not cheaper than you can locate another third member. I will say I have filed the caps for fit on a few 8 3/4 to tighten them and still got the threads to rotate the adjusters, but I used those on stock street applications, never tried it on something to be raced. To answer your question, you need to bolt on the caps measure the bores and see if the adjusters will rotate freely, if bore is off too much and/or adjuster will not rotate they are scrap. I usually look for .002 to .004 interference or crush on the bearing races.

Seems to me that the bearing saddles in the housing locate the bearings/carrier, the caps just clamp them down. If you're fairly sure that the correct caps are part of the lot, I would drag it all home and check the fit on everything using a dial bore gauge. Also verify a flush fit at the parting line.

All fixable

I installed my own Mark Williams billet steel caps by using a local machine shop Sunnen rod reconditioning mahine, I had to remove around .0090 form each cap to get them within Mark Williams provide specs
I put both sides on and that dang rear end still cracked the outer edge of the ring gear on all the gears sets I ran in it
I ended up bending the housing bad enough to have to replace it with a Dana 60 housing cut down to use in that car, that was the best thing I did to the rear of that car