I put a Chevelle rear under my '55 1st Suburban. 62" wide with 3.08 gears.
With a stock SM420 (granny gear 4-speed) it runs down the highway just fine up to 70 or so. It has enough other problems that I rarely run it that fast, but winding the motor is no longer one of its problems.
I used a disk brake kit from Chevy Duty (read other threads to get my negative opinion of them) that put GM mid-size disks on the front. I now have the complete braking system front and rear from a '73 Chevelle wagon, including the master cylinder and combo valve.
Changing only the rear and the brakes turned mine into a great driver. Of course on yours you'll be forced to change the transmission also.
