In my experience the original Stovebolt engines are dependable, I've put a lot of miles on them, some trips thousands of miles, coast to coast once.
Veering much from stock is asking for more trouble, dual carbs as mentioned, adaped automatic transmission, what else? All that seems simple and reliable enough, but they're changes and in general changes are either asking for trouble, or take some time to work out the bugs and become reliable.
A heavy truck, with a load, driving fast; perhaps just asking to much for what it is.
Have you ridden along for a road trip, is it just being flogged trying to get the job done?
