Usually 350/350 trucks came with 3.07 gears. With 31" tall rear tires it would probably be decent for longer trips.
My 3.07 geared truck with 29" tall tires is pretty comfortable at 65 mph, but not 75 IMO.
The single thing that makes late model vehicles more driveable is the overdrive transmission. I would be looking at going to 3.42 or 3.73 gears with TH700R4 for maximum value. 3.73 is a common ratio in 6 cylinder trucks so you may find a complete used rearend fairly cheap. And budget TH700R4 can be done for $1250 I think if you shop around.
Swapping a 427 into my truck and will be running 3.73 and TH200 4R because I like the gear ratios better. But it's not going to be $1250....
