There is a guy here in NZ with a '40 fitted with a 450 hp chevy. He gives it hell! It has a R30 5-speed gearbox and over many years of abuse has given no problems whatsoever. The R30 has a 4-spider diff unit so that would well contribute to low failure rates than say the R21Turbo box that has only 2. Other than that, the gearsets are virtually the same - only a few small ratio changes but yep it would work fine.