i don't know if 318 caps fit, but if so wouldn't it be cheaper to get a 318 block with caps and use those on the 340 block? obviously not 4 bolt caps but unless it's a race build they're not really needed.
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You would think you could pull set of caps off of a 318 and throw them over on another block and have it align bored and honed. But it just doesn't work that way because the caps are mated to the block differently between blocks. As in the Caps maybe side shifted on one block compared to the other. Then you can either weld up the cap where it joins to the block and file fit the cap over or get virgin caps and fit them.
Plus 318 caps are smaller than 340 caps as in less beef.
Last set of caps I got were those four-bolt main ones like in that picture in that box. I think I got them from Rocket here on Fabo.