340 main caps

Op might have to find a couple sets of caps set them on and see which ones lineup the best, see which ones are even close to snapping in and lining up to the bolts.
Go with the ones that line up the best and massage ,file, swedge, the registers to accept them...you will need a dial bore gauge to this properly....you will only get them close, not perfect, until its honed.... you will focus on getting an even amount of lip if lip is what you have or recess if recess is what you have once you get those dialed evenly side to side... then you can take it to the Machine shop for a line hone IF you did it right.

You can use 318 caps...but as already said, they are shorter by 3/8 or 5/16..but will still work...stud them if you want. The main problem was the no.1 main 318 cap likes to crack under big power/rpm.