Master Cly. Heat Shield

Aluminum conducts heat much better than steel. Heated air in the engine compartment will eventually transfer some energy to another surface. Air is a good insulator, especially if you have some venting going on in there. Radiant energy is the greatest problem, though. Those pipes get real hot. You can feel the radiant energy by moving your hand closer to the pipes. It decreases the farther away you move from the source. Radiant energy can be reflected somewhat by a shiny surface, but it is not complete. The surface will eventually heat up and conduct heat away.

My '68 GTX came with an aluminum foil-covered packet of insulation that sat between the lifter valley tray and the intake manifold. I don't know how effective it was, but it was there.

Anything that you can do to reduce the amount of radiant energy from directly shining on the master cylinder from the pipes is better than nothing. Radiant energy transmits line-of-sight. Fiberglas is a good insulator (asbestos was better, but that is another story). Remember, it is material with tiny air pockets that actually slows heat transfer through conduction. Therefore, a shiny metal shield backed with a Fiberglas pad might be the ultimate remedy. Same would go for the starter motor.