yes, the "L A " series of small blocks used a different bolt pattern for the tranny. In some applications, the big and small blocks used interchangeable "spool" motor mounts.
Depending on what you wanted to do, a 360 would probably be your best shot. If you wanted to go to a big block, the 383 & 400's are "short deck" "b" series motors and are slightly easier to swap in most cases. The "rb" 440 is just a bit taller, so it ends up being wider. In a truck, this would be no problem. Swap parts are available to put a big block in a 4x4 mopar from Schumacher Creative Services. The have all kinds of swap stuff, and I think the kits to do a 4x4 swap is about $150 to $350, depending on what all you need. ( www.engine-swaps.com
)You would still need to get a big block bellhousing, but they are not that hard to find used and are available new from places like Lakewood ( www.jegs.com
The 400 was a "smog" motor and is not all that desirable in stock form. The compression is very low and the pistons set very deep in the cylinder bore. It is easily upgraded. A 440 would be the ideal for a truck. In my opinion, the 440 is the best bang-for-the-buck period. For about what you would spend on a 350 chevy, you start with 90 more cubic inches. My low dollar 440 puts out 470 hp and 550# of torque and is very driveable. And the deep skirt desgin of the big block chrysler makes the more difficult to blow up, and keeps them from being destroyed when you do spin a bearing or something.
That four speed should be a very heavy duty unit with a granny low first, similar to a sm465. It will work with a big bolck with the right bellhousing.
I noticed that there is a complete rolling chassis big block 4x4 mopar on the off-road.com classifieds right now
Boy, I'm long winded today.........