Most likely the carb fuel bowl is going low or empty.
"Bog" would be a transition issue going quickly to full throttle.
If it takes off fine going to WOT pulls hard then has issues at high RPM's or top of 1st or 2nd gear it's another problem.
The easily cure is to install a high-flow N/S assembly in the factory Q-jet. All of my kits for the Pontiac carb come with them. If that doesn't fix the issue start looking at a restricted fuel filter, fuel pump, collapsed fuel line, loose clamps, etc.....
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73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
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