HELP: School me on 2 speed rear end ratios

[Bigdually]

I'd definitely be looking into an OD trans or swapping the center section to a higher geared setup if I wanted to keep the rear out of this truck. My GMC had 6.17s originally and it was wound tight at 55 in 4th.

I've been on the hunt for a 427 for a long while now. I had an older guy who'd owned a few MD trucks tell me he always felt that a 427 truck ran as good loaded as a 366 truck did empty. I don't know there will be *that* much different but I'm sure the power gain will be noticeable. Not to mention this 427 is about 100k miles fresher than my poor old 366. Plus the very worn SM 465 shifter in the GMC has so much slop it wiggles like it's in neutral when it's in gear