[Mike C]

For one thing, you have pretty high compression for iron heads and today's crappy gas. Flat tops in a 355 are about 9.8:1, the dome probably has you about 10.5:1. Especially high in a heavy vehicle. A lot of other questions to answer as well: Are you jetted even close to correctly? What rear gear? How tall a tire? How much camshaft? Specs? What is your initial timing? What is your total timing? How much is centrifugal and how much is vacuum? Is the vacuum through manifold or ported?

It is possible to tune for a heavy vehicle with high compression, you need to do an in-depth search for info on setting ignition timing at (a) initial (b) at a particular rpm and figure at what rate it gets to the total mechanical advance (c) how much vacuum advance and is it to smooth out the idle or to get to the proper total advance.