With a load and the original engine I doubt you can or want to go any faster than it will already go.
It was a new truck once, and built to safely handle the designed load, although not as fast as we are accustomed to today.
So after checking and rebuilding the brakes and adjusting the steering gearbox if necessary that's about all there is to do other than use it.
If you want to go faster you'll need more power and then better or more reliable brakes, it can be a slippery slope of modifications.... before you know it you'll have a diesel engine and who knows what else...
Grigg
