Captainkirk's Duster project

Just to be clear...
I can never remember stuff I had all figured out once-upon-a-time. Not like where the bathroom is, or where I put my car keys last (I got a system for that!) but essential stuff like gearing, cam lift, specs and such; stuff I had all figured out at some time or another. One of those things was differentials. I had it all clear in my mind what the rear end ratios were, and all that. Seems I've forgotten. So after re-hashing all this on another thread, I was given some valuable tips and tools; like this gear ratio calculator tool...

Gearing Calculator | RPM and Tire Diameter to km/h - mph with Shift Tracer

Using the data I can plug in, it gives me what tire sizes and differential ratios I can plug in and see things such as top speed, shift graphs, etc. So when I plug in things like a 4-speed trans and shifting at 6000 RPM with 6k being max (assuming a hydraulic cam), I can see (theoretically) that my top speed with a 3.55 rear axle would be roughly 140 mph. With a 3.91, we are looking at roughly 122 mph...assuming 26.77 tires. Change that, and the numbers change. But it gives you rough data to work with.
That being said, I couldn't recall whether the rear end that came out of The Duke was 3.55 or 3.23, and if it was a Sure Grip or not. Another kind member here gave me the info I needed to confirm this. So I crawled under The Duke this afternoon and checked a couple things, and I'll stick 'em right here in this thread where I can't lose them.
1) Both axles turn in the same direction. It's a Sure Grip
2) Roughly 3 and a half turns of the drive coupling gives me 1 revolution of the wheel flanges, so it's definitely a 3.55 and not a 3.23. There; now I can't forget again (unless I forget I put it here)
The 3.91; I've forgotten that info but almost certain it's a 3.91 Sure Grip. But it's buried in the corner under the workbench and I didn't feel like moving all that crap to get to it, so...I'll jot that info down here as soon as I get it. Given the above calculator, I'm wondering if 3.91 might be wrapping it it little tight? Not that I'm planning on going 140 any time soon, just wondering.

EDIT: I knew what was gonna happen if I didn't venture forth into the chilly House that Duke Built and check that rear end out of the minx. I was gonna get an earful next time I dared poke my nose in there. So I threw on the jacket, trundled out there, and began moving stuff around. Honestly, it didn't take long to determine that, Yes, Virginia...it's a Sure Grip. And a 3.91 at that.
The Duke never uttered a word. But I'll bet he was smiling.