66 gmc v6
This probably the 305 cu in v6. slow revving, low rpm peak torque truck engine. You could probably go out and buy two or three old big ol engine transmissions from from trucks or cars and not make the cost of that rebuild.
The main problem is, these days, parts for that engine are not that common.
Lots and lots of ways you could go. The big question? What can you afford? Set a budget and work from that.
so, what is the budget?