Years ago I put together a 55.2 with a big block. I used a stock truck bell housing. I don't recall the firewall interference, so it must not have been much. The headers were a bigger challenge. I am not sure which headers I used, probably Camero. The rear two tubes had to be removed and reworked. It is tight, but do able.
I used the stock small block front engine mounts. I do not think all big blocks are drilled for these mounts.
If you are going to use an auto trans, then none of this is relevent. You should side mount the engine and rear mount the trans, in doing so the engine can be located so that everything clears.
