I like to get limited slip fro my Dart and keep the small bolt circle wheels. Options are 8 3/4 which is expensive (here) and the 7 1/4 but I need a limited slip differential. Can I use a part for later Mopar and where I can order one. I have 3.55 gears which I like to keep there.

Yes you can use a 7 1/4" sure grip out of a later mopar, but you will be looking for a used SG unit regardless. They are hard to find nowadays, you would have to post to the parts wanted section and see if someone has a late-60's/early 70's unit they pulled from a spare rear-end.

-D.Idiot

I have a 1971 Valiant with a sure grip. It has 69k on it.
I will be changing over to a larger rear, but not right away. I don't know how much in a rush you are in.