SBM 340 Rear Main Cap

I have a 340 block with no mains. I was hoping I could use 318s. My machine shop just informed me the rear main is too short.
Nobody makes one. Anybody have any suggestions? At least, any help on specs and dimensions to have one made?

First post, have mercy, thanks
Do not use 318 main caps they're not beefy enough. 340 caps are all taller. Look for aftermarket caps