I had a 1977 b200 Tradesman. The big block was an option so Chrysler made motor mounts for it.
I bought the motor mounts from the local dealer. I dropped the engine and tranny out the bottom. Took the suspension apart, lowered the van down to just off a furniture dolly, unbolted everything and then jacked the van up high enough to roll the engine and tranny out.
Cleaned up the cross member and repainted it. bolted the 440 to it and rolled it back under the van with the tranny bolted up. I was only able to get it in with one header on. Put the other one on with the engine in.

I adjusted the 318 throttle linkage and kick down linkage bending, etc. to get it to work.

Really a pretty easy swap and surprises a lot of people when they get beat by my family room on wheels.
I did this over 20 years ago, so I don't know if you can still get the engine mounts new.

Also, the rear end is marginal as we broke some spider gears in ours.

Hope this helps.