360 year differences

I wouldn't worry about the heads, all 340/360 head castings until magnum heads have the same port size shape and will flow the same amount of air. Only difference would be some of the J heads had 2.020" diameter intake valves which certainly aren't needed if your goal is 13.00 in an early A-body.

Also the advertised horsepower difference between 71 and 74 is a function of the change in the way power was measured not any changes that were made.

I would assess each engine and take the one that is the most complete or appears to be in the best shape. In either case they will likely need to be rebuilt .