OK I looked up the movie to see the truck.
Looks like 7.00-20 dual wheels. Nice big rolling radius. My truck has the same.
If you have a 5.43 diff, you will be fine. Standard its doing 3000 at 60 mph. A 6.17 is that much worse.
With .8 OD you'll get 2400. OK for a bigger motor. The original 216 probably wont pull it without a lot of throttle.
Anyway the diff. The front pinion bearing is uncaged needle type. I dont know a lot but I think these are designed for load not speed, with revs they tend to bind and grind. That said, they were designed to do 50 and 60 aint much more. Add in modern oils and I think you will be fine if you dont exceed 60 by much.
All the other bearings are monster size, barrel type and I dont think they will mind the extra speed.
From what I have seen the diffs never wear out and this would include any abuse they ever got.
I plan on running mine all day at 60, and I dont expect issues.
If you get serious, clean out the rear axle housing- my diff unit was clean enough but the housing was full of old hypoid gunk, like about 1/4 " of it right through. Running it faster may dislodge this and let it float around.
