Another option for you to think about is a two-speed rear. I now this will work as I've seen it several times as a factory GM option behind some junior sized Allison's.
(even in school bus chassis if you can believe it)


1953 Chevy 5-window 3100
In the Stovebolt Gallery
More pix on Picturetrail

Dave
Engine & Driveline Moderator

If you can't make seventy by an easy road, don't go. ~~ Mark Twain