Frankly I'm surprised you're replacing the entire engine with such low mileage. I know it has some issues but you're replacing it with a used engine.

You stated that you're going to:

1. Replace all gaskets and seals
a. front timing cover seals.
b. rear main seal.
c. oil pan and oil filter housing gaskets.
d. intake manifold gaskets.
e. head gaskets.
f. valve cover gaskets.

I'd view this differently if you were going to just bolt in the replacement engine and go...But if you're going to tear down the replacement engine that far it seems to me you could do the same to the engine you have and figure out what the problems are with it and fix it...Vibration problems and all. Now if you have different goals in mind and you just want to have a 383 project on the shelf while you drive around using the replacement motor...That's different. You've had a few problems with this engine and its given you the excuse to drop in another...Well okay. If I had confidence in the replacement motor I was buying I'd drop it in and not look back...Otherwise I wouldn't buy it. Drive time is better than down time. Either way you decide to go I'm sure you'll be happy with the outcome.