I think it's getting vapor lock when the heat soaks in, it's a good and bad thing, the good means your fuel pump can keep up with the heat, the bad is once it's vapor in the gas line it takes a bit to cool the gas down to pump liquid again.
you can replace the fuel line with a braided line or place a rubber line over the metal line, keep the gas line as far away as possible from the exhaust.
if you have a manual choke then disregard this. Another possibility is the exhaust manifold heat tube for the choke has given out, and exhaust is being pumped directly into your heat tube for the carb choke, if it's an auto choke. the exhaust manifold hot air pipe is known to fail over the 40+ years of service. to check if yours has failed, use a gloved hand (very thick glove cause it will be hot!!) and place it on the hole between cyl's 3-4 on the underside of the exhaust manifold, then with your ungloved had, bring it close to the hole on top. if you don't feel puff's of exhaust then your safe for now. if so, I recommend a new exhaust manifold and for you to service your auto choke as some of those parts can be melted.
Hope I helped and if you find anything else let us know we are happy to help.
