whats my Horsepower gain with these mods?

Putting a bigger carb on a SL6 doesn't make a lot difference if everything else is stock. The Super Six (two barrel) made a little difference but not a real big change. the effects of increasing compression on the /6 makes a huge difference. Any where from 9 to 1 to 9.5 to 1 really wakes up the motor and that is leaving everything else near stock.

Performance upgrades that make a big difference.

Compression increase #1
advancing the cam or having yours reground for more torque and advancing it #2
#3 is re-curving the distributor.
#4 would be going to 2.25" exhaust and a free flowing muffler
#5 would be going to electric fan to save some HP. The electric fan has many benefits besides increasing HP. It lets the engine warm up quicker, and reduces heat soak for better mileage.
#6 upgrade to a bigger carb