A quick thought, you say that you are running a 350 with a 700R4. I to have this combination. Sooo, it would seem to me that your drive shaft had to be shortened. Are you sure that it is not out of balance?
Beyond that I also installed a rear end out of a 71. They bolt right in with one change. Can you weld?
The panhard bar bracket on your 66 rearend needs to be removed and put on the donor rear end (there are specific steps for this). (I believe a rearend from a 67 to 72 will work and only need this mod) (73 and up needs the trailing arm brackets relocated as well)
This will give you 5 lugs.
