Well I don't want to sound like a pre-Christmas Grinch, but you're talking about a lot of work here.
And its not just the parts swapping you are talking about, but fully answering your question...
The challenge with a general question like this is that most people who might respond only know part of the answer at best. You might do better to ask specific questions in forums where the people who know are likely to be watching.
Mounting the 350/350 combo in your '39 isn't a real big trick. The most challenging part if the steering box/exhaust interference issue.
However there are power steering conversions for '47-'59 trucks that use the steering box from your '77 parts truck. You might be able to find a similar kit for a '39, or perhaps adapt the later kit to the '39? Converting to power steering that way fixes the exhaust interference problem.
The power brakes on the '77 proably use a firewall mounted vacuum booster. If so the booster is too large to mount under the floor on the '39 and would be ugly mounted on the '39s firewall.
The track width on both ends of the '77 are too wide to fit easily under the '39. Takes a huge hammer to make them work...
As for the bed floor, ask in the body work forum. For the guages and wiring harness, drop those questions over in electrical.
