74 Slant 6 Bogs & Stalls Under Load / 30 mph

For a motor to run it needs: air+fuel+ignition.
If the engine is stalling during an idle or while being driven one of those is dropping out.
Some simple analysis could point you in the right direction.
Right after it stalls, remove the air filter housing and with the engine still off look down into the carburetor and open the throttle fully with a quick motion. You should see a healthy fuel squirt into the carb venturi. If you do, that’s good, if you don’t that means that either the accelerator pump is mid adjusted or defective or the carb is running low on gas. Running the carb low on gas is normally a weak fuel pump or it could be a clogged line or plugged filter.
If the accelerator pump flow is normal, and the air filter looked good when you removed it, one more way to check for a fuel problem is to remove and do a visual inspection on the spark plugs. Very black plug ends is too rich, very white plug ends is too lean.
If the plugs also look good, you can have confidence the issue is not fuel or air related and is likely a spark-ignition issue.