You don't need to find a econo straight 6 any small 6 from a 144-200 will drop right in on the current motor mounts even. look for falcons,mavericks,fairlanes, plus other fords that had these engines. I had a 65 with a 70's 200 from a maverick. I would also consider when jumping to a 302 that you consider all the space you have to work on your engine is eaten up by the size of the 302 or at least thats what it looked like to me from the pics i've seen.