[D13]

1982 250 3ott 3.43 geared 3/4 ton, 80 is absolute limit.
1987 6.2 diesel 7004 3.08 geared 1/2 ton, fastest is 75, truck felt rock solid, issue was the noises under the hood.
I would suspect with a reasonable motor and OD trans with good gears that 120 would easily be possible. I read somewhere (Jim Feuling in CC in the 90's maybe?)that if the tailgate is open over 120 or so the vacuum bubble will close it - so I sure wouldn't want to try higher speed corners without some rear down force.