Nice to see a beautiful truck...great job!
Performance Distributors / DUI HEI vacuum advance distributor ?
Did you have the distributor advance curve tailored to the truck, gearing etc?
Sounds like its a bit slow to bring in the advance....
Misfire under load could also be a plug lead arcing across to a ground...maybe to a header pipe....in Park try looking under the hood in the dark and blipping the throttle....darkness makes the spark more visible...
I would be looking more at the distributor first...including the module...
