here is another guy that has documented a SBM into a Mopar. Nice stuff. I hope that it will help those that are considering updating. They are .pdf files that you can save.
http://www.jalopyjournal.com/forum/showthread.php?p=4534581#post4534581

Just wondering out loud here, but if a small block can be fit into the 40-early 50s cars by offsetting the engine, using a truck pan/pickup and it fits, why not a big block? They stuffed them into A bodys, so it may fit. The exhaust manifolds for BB darts aren't cheap, but this might help.
http://www.bigblockdart.com/techpages/HPman.shtml

On my 40, the main areas of concern were the steering and right rear valve cover to firewall.
The radiator to WP clearance does not allow a fan, so I have to run a pusher. The big block may be too long, but a custom radiator could fix that.

Doesn't the plymouth have a slightly longer front end than the dodges, or was it vice versa?