Ford 8.8 swap?

If you don't already have a 8.8 another option that I think is the correct width is a 8.8 out of a 90's Ranger. My Pop has a 64 Rambler that someone installed a Ranger 8.8 in and I measured it and was surprised that is 56-1/2" wide flange to flange which is just 1/8" difference from our a-bodies. The driveshaft offset was 1-1/2" which is 1/4" difference from an a-body but on Pop's Rambler it doesn't vibrate and it calls for the same offset as an A-body Mopar. All you have to do is cut off the Ranger brackets and weld on a set of Mopar spring perches 43" center to center and slap it in.

Your tire height will have a great effect on the final drive ratio which determines the rpm your engine will spin down the highway. Here's a calculator that you can input all the values for your car and it'll tell you the rpm's it'll be spinning down the road

http://vexer.com/automotive-tools/speed-rpm-calculator