Quadrajet 350SBC Tuning Advice

[burnin oil]

A couple of thoughts about this. First is if this is not a new problem the qjet may already be on the secondaries so it lacks that punch. This happens alot on low geared trucks since the primary are so small. Secondly double check that the choke linkages are working correctly. They may be keeping the air door from opening under load.
Your timing curve sounds pretty good. I second the manifold vacuum for the vac advance but you need to try both ways. My RV cammed 350 actually likes ported vacuum. I also agree that 20 degrees of vacuum is probably to much advance for a truck motor. Usually the adjustable advance cans adjust through the vac nipple and only control tension. This means it only adjusts the amount of vacuum needed to have vacuum added. These cans usually require a limiting tab to control the degrees of advance. The crane kit has decent directions on the usage.
A sticking timing wieght can cause all kinds of issues.

If you get a new fuel pump be sure to check the pressure. The last parts store stock replacement I bought had to much pressure and constantly lifted the needle of the seat causing it to flood. Needed a cheap regulator to fix that issue.

If the carb is good and just needs more fueling, you should be able to use secondary metering rods from an edelbrock carb since they are clones. Q jets are 750 cfm but the air door design only let's it flow what the truck needs. Yo may only be getting 500 cfms of air and need some richer rods. I have seen qjets run well on a mild 283 without being over carbed. Great carbs with airplane gears but can be a mismatch in a low geared heavy truck. My 1 ton with 5.14s is always on the secondaries and felt weak. Swapped a used edelbrock carter clone on after 1 trip. Way better manners. I hope all this made sense. My phones browser isn't letting reread this post for clarity.

As an afterthought. If ignition and carb tuning goes nowhere, pull the valve covers and check the cam. Intake lobes going flat will cause power loss under load but rum pretty good otherwise. Exhaust lobes going bad are the ones that cause back fires through the carb since the increased exhaust gas can't get out of the cylinder.