You wouldn't have to mess with the master cylinder for the heat shield. Just get a flat piece of sheet metal, bend a 90 deg bend in it, drill a hole and bolt it onto one of the master cylinder bolts.
We did a similar thing on our Baja car in college. Per packaging and the rules we had to have our gas tank mounted above the engine. Unfortunately, our fuel line that came underneath our gas tank was got hot do to the engine and was causing the fuel to evaporate. So, we just placed a thin sheet of metal (titanium to be specific, but aluminum or steel would do) as a barrier between the engine and fuel line and we stopped having fuel issues.