If he has stump pulling gears, he won't get down the road very fast without an over drive.

With my 58 with 3:90 gears in 3rd about 60mph is a fairly comfortable speed. Any more the engine sounds like it is getting the rpms on up there.

With the 50 I had with the same engine and transmission but with the 4:11 gearing about 50mph was the comfortable speed. Above that the engine felt like it was revving pretty high.

In either case he does need to make sure the engine is tuned right and running right before worrying about what the transmission is or isn't doing. With the TH350 if the vacuum source isn't good, or the engine is running poorly it would mess with the shifting of the transmission. So the engine has to run right first. Then see if he really has a gearing issue or not.