Not crazy!
There's always the debate as to whether it's an actual proportioning valve or just a distribution block, so assuming it's basically a distribution block I'd go for it. The biggest difference in the master cylinder (assuming the new and old are the same bore diameter) is that the disc master won't have a residual valve in it for the front discs, and those are leftover necessities from when masters were mounted low in the car; most agree that they're not necessary anymore for drums, anyhow. And you'll have a bigger reservoir for the front brakes.
Line pressure should remain the same if the master's bore is the same, so my bet is that you won't even notice a difference in braking- plus you'll have a safer dual pot master now.
