Casey,
Sounds like you got the serial number almost right...
6A = 6100 series (132" wb)
58 = year
S = St Louis Plant
123241 = unit number starting with 100001, so you have the 23241st unit out of St Louis
The actual build date is not available from the chassis serial number.
This truck would have had the 261 6-cylinder as standard issue with the 283 V8 as optional.
The 2-speed rear is a 6.40/8.72:1 ratio.
Are you running 8.25 x 20 or 9.00 x 22.5 tires?
Making the truck run 65-70 on the hwy will take the truck beyond it's safe zone. The steering and brakes were never designed for those speeds. Your SBC should push you along nice in the upper 50's - low 60's mph range.
If you really need to go 65-70 you will need to swap out both axles for better gears and better brakes. My '57 Fire Engine will run 65 mph on a flat road and has been 70 going down hill, but that's WAY over my confort level...it gets real squirrelly at that speed.
Rear axles are easy to swap, for the most part they are all the same width. The front axles are a different story, you'll need to measure and search for a good fit.
Mike B
