8 3/4 rear in 65 Signet
As others have stated you will need to shorten your drive shat, plus you will need the shock plates for a 8 3/4 and the u bolts. Another thing to keep in mind is the bolt patern for your wheels. 65 had the small 5. Now depending on what year the 8 3/4 is your bolt pattern might be the 4 1/2 on 5. I believe up to 73 are the small bolt pattern. This might also help you out.
As others have stated you will need to shorten your drive shat, plus you will need the shock plates for a 8 3/4 and the u bolts. Another thing to keep in mind is the bolt patern for your wheels. 65 had the small 5. Now depending on what year the 8 3/4 is your bolt pattern might be the 4 1/2 on 5. I believe up to 73 are the small bolt pattern. This might also help you out.