I do not know if anybody else is aware of this but here goes, I have a 69 Valiant with a slant six k member and I am in the process of building a 340 for it, anyway I was messing around in the shop and I took a 340 block and associated brackets and started mocking the block up.
This is what I found, using the correct drivers side mount(340 block 340 bracket) and on the passenger side I used a van or truck mount and it dropped right in, now this was only the block, my next step is an engine and trans.
I have some pics but they are not great.