simplicity question

ok here i go . well I have had a extra 7100 frame sitting aound and just picked up a 3 speed rearend off a 3414 and got it all together looks rough but I cant paint with no heat in my garage now i want to run a tiller in summer and a blower in the winter but when i set both on it the belts seemed to long. So is there and difference between the 7100 and the 3400 rearends?????????