I'm with Peter.
Sounds like a fuel starvation problem. If the only fuel pump on this vehicle is the mechanical on the motor and the tank is at the rear it's not enough under heavy load. The engine will consume fuel faster then the float bowl can be filled and it will basicly run out of gas under throttle. A second electric pump close to the fuel tank should be installed with a safety switch. THe safety switch is a oil pressure switch that if there is no oil presure it turns off power to the fuel pump. Standared switch used on GM vehicle with carbs from 80 up. On those vehicle is was used to cut power to the electric choke.
The electric pump should overcome any possiblilites of vapor lock. In a nut shell vapor lock is where the fuel is boiled. It either expands after the fuel pump and over comes the pumps output pressure or it boils before the pump and the pump is trying to suck air and it can't create enough suction to pull fuel from the tank. A remote pump will push fuel to the pump and then the return line would come into play and send the boiled fuel back to the tank to cool. It would then move enough fuel to end the problem.
Next question is if the secondaries are working correctly. On the Quadrajet the main bores are rather small so that you can get fuel echonomy and off the line power. The secondaires come on as engine needs it. You could put your foot to the floor and if the engine isn't moving enough air it will simply not open thesocondaires. That's why the carb off your 454 could be bolted on a 305 and run near perfect. The adjustment is tricky and since the last quad factory was over 13 years ago tuning these carbs is a forgoten art.
A mis adjusted choke could cause simular problems because it has a lock out that does not alow them to come on till the choke is FULLY open.
ANother problem is overtightend bolts holding the carb both together and to the intake can warp the the carb body and jame the flaps on the top for the secondaires.
Last possiblity is vacuum leaks. Actully this should be the first thing to be checked for. it would cause a lean out condition that would lead to detonation. The engine would start to sound like a wood pecker. (non opening secondaires would cause same symptoms). On GM motors of that era they also used several vacuum switches. on the water nect may be a unit that is temperature contrled valve. When cold it would lock out a vacuum pot on the side of the carb that alows the secondares not to come on. On thses versions this is the second secondary lock out. Also the same device is used to force the secondaires to close if you get off the throttle to help prevent a over rich condition and exhaust back fires on deceleration.
So basicly any vacuum problems could be causing the same problems. Also don't just look around the engine. The HVAC system uses vacuum for control flaps. Brake booster also uses it. A problem with the EVAP system could also be causeing a problem. If not working poperly it may be placing a vacuum on the fuel tank. This vaccum could overcome the fuel pump(s). A quick check would be with a low tank (so fuelk doesn't pour out the fill) leave the gas cap off.
Another thing to check is the Air cleaner housing has a pree heat flap that pulls hot air off the exhaust manifold called a heat riser. It aids in cold start and cold weather it may partly open to preven frost from forming in the carb bores. Once warm it should be pulling cool air from the main inlet.
If that were not to function properly then two problems are caused. One your sucking hot air. Two the inlet from the the heat riser is rather small and it absolutly cant flow the amount of air the engine would require under load. It would basicly choke the life out of the engine. \
Your unit should have the full vacuum control version. If you pop the air cleaner off there will be a small vacuum line pluging into the carb. Pull it and plut the place it was connected. This will make sure it doesn't open.
THis could also be an ignition problem. I doubt your unit has ESC (Electronic Spark control). It was a deal where the ignition system had a Knock sensor and would retard the timing if it heard detonation. I am pretty confident you have HEI. One thing that sometimes happens is the mechanical advance in the dirstributor will get hung up. The gasees from the crankcase will some time go up into the distributor shaft. The mechanical advace rides on a second shaft inside the first. those gasses will form deposits and coke up the advace causing it to hang or not funtion. The distributor needs to be checkd for this.
Ok now you probably know as much as your mechainc about what you have. Go bury him and make him work for his money.