Something to keep in mind. Older differentials like that on my 78 and possibly the 74 are limited slip 456 rear ends. They have better acceleration than the latter models which were more geared for fuel economy. My 92 has a limited slip 411 rear end.
I'm planning on a V6 conversion and thanks for your reply. I'm aware of the ration difference and it's still unclear to me which ratio is best to have. However with the way Nick Muto's car smokes the rear wheels, if he kept the 4.56 in his 70's Spider, my thinking is there would be benefit to a 4.11.
I agree. A V6 conversion and the 411 rear end would make a better combination than the 456. You would be turning too many rpms too quick w/ the 456. Be sure to post some pics of this monster after you get the conversion done. Should be a blast to drive!