It should be a drop in replacement for your 360, if it's a complete long block.

Many of the parts are interchangeable. But, with 600 miles on the rebuild, there's not much reason to use any of the 360.

It will be a very dependable, torquey motor. It will not be a a high revving motor, nor will it get good mileage.

The blocks are basically the same. the motor may mount differently, but if you have all of the parts to make the 360 work then you should be able to make the 391 work.

A couple of the differences I can recall: the crank snout is larger on the 391 and the disitributor shaft is a different diameter.