Exhaust!

[really?]

At the end of the day, a muffler can only put out what it gets in. A 305 with a 2 barrel is a pretty weak sounding motor, sorry to say. You never stated what size your Y-pipe is or what exhaust manifolds you have. The exhaust system works as a whole, not just a muffler alone.
A dual 2.25 inch system is a good idea, I believe HOOKER sells a system pretty cheap. I don't think it comes with an H-pipe so it will be wise to add one! An H-pipe will clean up the tone and increase HP and TQ. Better to go for something that sounds good at a lower tone than just loud and ****ty.
Make sure the Flowmaster you have is a legit Flowmaster. ALL Flowmaster mufflers come stamped on the downseam, gotta watch out for those knock off companies. Oh yeah, make sure the flowmaster inlet is facing the motor, you'd be surprised at how many people think it's ok to turn the mufflers backwards lol there's a reason why it says 'INLET' on the front.