Hey stevebee,
I don't know much about that Alpha, but I am familiar with small blocks.
50MPH is a solid number, if you are hitting it. Your prop slippage with those numbers is only 7%. Very low. Is your speed GPS? If it is, then I don't think you'll get much more by spinning the motor higher, as I am guessing you are right in the top HP range for that engine. However, with an aluminum prop, I'll bet those numbers are suspect. http://www.goldenplasma.f2s.com/foru...confused14.gif
As for horsepower with that drive, I think the general rule of thumb is 300HP, however, I am pushing over 450HP through my Volvo (which the Alpha was designed to the same specs). IMO, a driver will kill a drive faster than horsepower (or more important, torque). http://www.goldenplasma.f2s.com/foru...s/scared04.gif
To make more horsepower at higher RPMs, you'll have to make that engine breathe easier. If you change the cam, go with some good heads and manifolds with through transom exhaust, you'll be able to spin that small block higher (more HP). For a starting point, I would look at the carb and manifolds.
http://www.goldenplasma.f2s.com/foru...es/happy25.gif