So which bolt holes line up to your bell and do they all line up when fitted onto the dowels? Only C4 bell that fits those early lower block pattern holes is a C4DP-7976-A. The C5GP-7976-A bell fits the higher bolt circle in your picture. You did say the trans was out of the 1970 Mustang do you know which engine was in it? 1970 they had two sixes to chose from vin and door code T is for the 200 cid 6cyl (C5GP-7976-A bell) would bolt up to your 1966 170. Code L is a 250 cid 6cyl. Though the engines look alike they do have different size bells 250 is the same as on a 302 V8. Lastly I guess that someone could have swapped in an older 1964 170 or 200 engine and bell (C4DP-7976-A) into that Mustang. This should be a very easy bolt together swap if I could id what parts your working with, so some pictures of the bell and all the other parts would sure help.
