As Dan says; not usually an issue.

A photo would help to allow people to comment on how “next” to the muffler the pump actually is.

Also, what sort of SU pump do you have. IME the old points based pumps do need changing about every 50 years or so.. ;> The modern electronically switched pumps are a different ball game ..again IME.

Cheers, Ian

No takers so far.
Usually heat is not a problem for a SU pump on the rear subframe. The usual position for the muffler is diagonally from the floor hump to the outer corner of the car, so there's lots of airspace between the muffler and pump. With the car in motion, the airflow passes the pump before the muffler, so the heat it would be blown away from the pump. Note the location in the engine bay of the fuel line in relation to the exhaust manifold or header - much closer and hotter!

If you are having pump problems, it is unlikely heat is the issue.

Does anyone know of a heatshield for the SU fuel pump next to the muffler? Thanks for any sources or custom ideas.