OK, one thing I can say for sure is if you have as much energy for your truck as you do for posting here you will have 500 ponies in no time.
The Stage-One can be built to 550 hp before a block girdle and other expensive parts are needed. This is providing you have used good rod bolts. Main studs would not be a bad idea either. The main webs are the weak point in this engine. If you build much over 550 hp and the mains can pull out. There are those that get away with 600+ hp with lightweight, well balanced rotating assemblies, but it is a calculated risk.
As you move from 500 to 600 hp the engine is less and less streetable. The compression needed makes the octane requirements higher, the transmission converter stall rpm will need to be upward of 4000 rpm and the vacuum will no longer support power brakes. I would say that somewhere between 500 and 550 hp you no longer have daily driver.
A solid 500 hp build needs 10 to 1 or better compression, a performance camshaft, heads ported by a Buick specialist, a good intake (stock can be made to flow OK for this), headers or ported exhaust manifolds flowing into a 2.5” dual exhaust system and a good HEI or better ignition system. Your Q-jet will work fine on this engine if it is rebuilt for it.
You will also need more stall in the transmission torque converter then stock. The performance cam will move the power range higher, and your converter will need to reach that range quickly. I think something close to a 3000 rpm stall converter will work. Your rear gear will also need to be 3.08 and preferably numerically higher in order to feel the benefit of the combination in the seat of your pants.
There were big port heads in 67 and 68? that had slightly larger intake ports, but they did not have Stage-One valves. They needed to be converted to Stage-One valves and did not provide a lot more hp then Stage-One head once converted. Head porting and matching the rest of the combination is the key to power in this engine.
Finally, a truck is just plain heavier then the nimble little pony in my stable. It takes less hp to power a lighter vehicle to the same speed as a heavier vehicle with more hp. Your local PIA Stang has you at a weight disadvantage. You will need the 500 hp and to put your truck on a diet to spank him properly. Then we can see about bigger fish up north.