I bought a Flowmaster 40 series to replace my main muffler (which I removed a few months ago) but it was a bit to big. I have to buy another 40 series so I can replace my dual mufflers. I'm looing for a deep, aggressive tone. In the meantime I have straight pipes. It sounds raw, especially when it idles, but after any aggressive acceleration if I take my foot off the gas too quick, it backfires (humiliating). What causes this, and how do I fix it before I get my other 40 series (might be a while)?

My guess is your removing backpressure so the engine is running a bit leaner. A backfire occurs when unburnt fuel is sent through the exhaust system and then ignites.

Here is a page of info on it...

http://faq.f650.com/FAQs/BackfiringFAQ.htm

Is it a flowmaster 40 for the f bodies? Or is it a G body/W body unit?

I was told directly by Flowmaster that I should go with a 50'* series, and warned that it may have a harsh, irritating sound from the supercharger and strange timing of the 3800. Try calling them...they're great...they might be able to help you figure out what to do.

There is no cheap fix unless you get a used muffler and a temp install (clamps).
The unburnt fuel and back firing can cause problems with the cat and valves etc and should be addressed soon.