I assume you are using the L67 blower? it should make more than 50HP. stock L67s from GTPs are rated 270 from the factory. that is 70 more than the stock 3800.
from there you can reduce the pulley size to add more psi i believe to get a few more horses. if you used the 3800 heads it could be where you are losing some hp the L67 has different heads which could be better flowing. what other mods do you have to support the SC.
Cam
Headers
Injectors
Tune
all those can add hp and greatly improve the SC gains. i'm running 42# injectors, cam, headers, and tune on a NA setup and getting close to 50 hp gains
as for another SC you can look up an holden m90 SC. i believe that will bolt up to the 3800. I don't know the specs of the sc but they will be very similar. if you have no supporting mods its going to be hard to push hp out of the motor. you will be better off using a turbo. even then the stock camshaft will only be so effective dealing with boost and you could detonate the block.