My thoughts on the /6 .... they are a great engine, they are getting harder to find today also.
I originally thought about a /6 because I have had several 60's Dodge trucks with the /6 and love them .... I settled on it would be a poor choice.
I think a 3.9 V6 from a Dakota would be a great swap. They have plenty of power and even a Magnum version that produces more HP.
I have a older Caravan with the 3.3 FWD, newer came with a 3.8 which is a stroked 3.3. The Dakota RWD trucks have the 3.9
All these Dodge V6's are known to go 300K + miles with basic maintenance.
Will require extra wiring with the computer .... should be pretty simple if you have the engine/trans wiring harness and computer from the donor vehicle.
While my original 218 is in good condition, I did pickup a 1970 318 for future use.
The 360 is basically the same block and the newer replacement for the 318, should be plenty of them around and make a good choice.
318/360 has been done thousands of times and work well .... just a better choice then a /6 IMHO
All deals are off if you already have the motor/transmission then go ahead and use what you have.