Water tank on the engine

[Scargo]

So a high temp pressure relief valve, in line to a catch tank, set at a pressure just under that of the rad cap should do it, much lighter and takes up no room. Thoughts?

high temp pressure relief valve? You mean a radiator cap? Subaru uses a higher pressure cap on the radiator so the expansion tank (Subaru calls it a water tank) will release first. About 1.3 bar or 19 PSI on the radiator and about 1.08 bar or 16 PSI on the tank. I went with both being 19. You can get 25-30 PSI caps if you want.