Andy man...
Thought you were going to run that thing down some weekend...
Where is your PCV valve and hose going into the intake/base of the carburetor?
What month/year is your 55?
Please post that.
Please clean off the serial number stamping by the starter motor and post up what you find. Should read: F XXXXXX
A true 1972 should not have an oil fill and PCV valve set-up protruding from the right side of the engine block, in front of the distributor, like what you have pictured here. Those were on pre-1972 F engines.
You say that it backfires under load; do you mean under acceleration load, or deceleration load?
And is it popping out the tail pipe or back through the carburetor?
Also-
It appears as though your exhaust manifold gasket is burnt/leaking at the cylinder head in this picture.
If your popping noise is happening when you decelerate, it is very likely that this area where the gasket is burnt away is allowing air into the exhaust system, which is likely combining with un-burnt fuel and this is causing your popping noise.
It is also very likely since the manifold gasket is burnt up, that you could have leaks on the intake side as well, which could also create a popping scenario, due to the un-metered air being introduced into the manifold, creating a lean situation that in order to get the engine to idle at all, would require the carburetor being adjusted to a very rich setting, which could very well be adding to the popping issue.
