Back in the day I heard guys say a TH350 took 100 horse and a Muncie took 50 horse.

Is it true if I get a stick I'll pick up 50 RWHP?

It's not that much, and it doesn't have to really do with the trans itself. A stick is usually more efficient because a clutch is pretty much locked up. A torque converter will always have some percent of slip, sometimes as much as 10% which will be the loss of power. I would say maybe 25 Rwhp at most.