Buick Rear End swap on a 60 Electra

I'm putting a Caddy 500 in my 60' Buick and i'm stuck cause i can't find a rear end that will fit. It originally had an enclosed driveshaft so I can't use the original rear ( Unless anyone knows if a 63-65 Buick Riv's guts will fit in my hogs head ) and the caddy's rear is 4 inches too long. I need a rear that is 58 inches from backing plate to backing plate. If you know of anything that will work I would greatly appreciate your feedback. Can't wait to do some smokey burnouts in the big body Buick!

 

79-81 Olds 88 rears fit.

 

Center section from a '61-'63 Buick rear bolts into the stock housing.

 

I thank you both so much. You have made my day. On the hunt I go tomorrow.