A used 3550 and bellhousing. You can that those bought and shipped from Ebay for around that price. Not sure what you have now so I'm going to assume a T-5.
They make super stong T5's. And they have better ratios than stock. Prices are in your general range.
From their website:
"Although we do not like to put torque or horsepower ratings on our products due to the vast differences in clutch, engine, chassis, tire, and mounting combinations, we can give a very rough estimate..............Generally, it can be assumed that with the available upgraded mainshaft, the G-Force T-5 kit will handle roughly 600 hp or 500 ft-lb. or torque in a 3300 lb. vehicle running drag radials at the track with a moderate clutch upgrade."
I would save up a little more money and get a tremec tko 500 I think it is...those things can take a freakin lickin. People in the mustang mags with 600-800hp are usin the 3550 so thats tells ya they're good. Do it right the first time