Narrowing an 8 1/4" rear end, how is it done?

In my 1965 Barracuda I am running a 1977 Dodge Aspen 8 1/4 with a Dodge Van's 8 3/8 3.55:1 Suregrip. With stock Cop Car Ralley 15-7 rims and 275/60 15's. I put the drums from my 74 Dart Swinger, 10" by 2.5" on the housing too.

The 8 1/4 Center Diff is the same as a Truck 8 3/8's so finding a set of 3.55:1 Suregrip is easier too.