Maximum possible speed really is a function of horsepower and with original type motors these trucks will never be speed demons. But run the math on the transmission and rear axle ratios and you'll see pretty quickly that it takes more than a motor swap to move one of these things fast.
I think your mechanic might be a little pessimistic, but only a little. On the other hand how fast do you really want to go with 60 year old brakes suspension and steering?
I'd like to know how you intend to use the truck, kind of roads, kind of speeds, kind of loads, length of trips, all that kind of stuff before I try to comment on what the right modifications would be.
