[Coley]

Ok....lets start with the 'real' start point.
What is your budget for improving the drivetrain issue on this truck?
ie: $3000? $5000? $1500? $1000 or?....this is the realistic start point and its good to be honest about this because it has a massive bearing on what you can do, obviously.
....this is job one...question one.

I appreciate the fact that you have the original engine...I would hang onto it.

However, what I would do is pull that motor and put it aside in storage for the moment and drop in a 454.
(you can take the original motor and have it rebuilt over time if you like...or keep it offer as being available for the next buyer)
The 454BB is physically identical to your existing 402BB so all your brackets will bolt right back on.
If you are interested in appearing original...just tell everyone it is a 402.

You will get a nice power/torque bump in that option.

As well, if you find a good used 454 (and they are out there) they often have their TH400's with them.
Its not uncommon these days to find guys parting out low mileage motorhomes that are selling of the low mileage 454/Th400's out of them...and some pretty reasonably priced.
If you go this route...ask to hear the motor, check it out, compression, do your homework.

All good ....and good luck!
Coley