Hi Ingo,
Since nobody else has chimed in, I will throw in my 2 cents. I daily drive a 48 - 3/4 ton with 4.57 gears and a 235 w/ 4 speed. I have 225 radials on back and I can run 40 without too much strain. I have run 50 or a little higher but its wound up pretty good. Your half ton will have 4.10's and should run a little faster. I would think that the Patricks gears would get 65 but I dont know at what rpm's you would be at. This subject has been discussed here in the engine and driveline shop many times. I know at least one of the sponsers or advertisers here has a rpm/speed calculator. You just enter your tire height and gearing, and it will give you rpm's or speed. Just do a search and see what you come up with. Maybe one of the wise bolters will chime in.
Ralph

rtlopez@shipleysystems.com
48 Chevy 3600 w/235
48 Chevy 3100 w/235:
You cant see the smoke on a dirt road.