Congrats on the purchase!

My guess is the "boiling/gargling" sounds you are hearing is more prominent with the air cleaner lid off, and engine running? If so....

Many of these big blocks used an aspirator tube. This was a low budget (ie: fairly ingenious) way around needing to drive an air pump. Fresh air travels through a small hole in the base of the air cleaner housing, through a tube to a 1 way check valve, then to the back of one of the exhaust manifolds. Exhaust system pulses draw fresh air into the exhaust up-stream of the converter.

The air pulses make a distinct sound when the air cleaner lid is off. If it ain't broke, don't feel obliged to fix it until you remove the catalytic converter. It costs you nothing in horsepower.