As Mike said, the Task Force trucks will run in the low/mid 60's as long as you aren't afraid to mash the pedal.
My '56 8400 tow truck has a 350 w/4+2 and my '57 10500 fire truck has a 322 w/5+2...They both run about 65 mph on the interstate.
There aren't any better gear sets available for the GM rears. If you find a truck with an Eaton rear you might find better gears if your lucky.
No self adjusters in the TF trucks...Not sure if or when they were available.
Mike B
