I think the real secret to flowmasters is to use some too-large for the application.

On my 78 bird with 305, I have a pair of 3" inlet/outlet flowmasters, they are obscenely huge. I've welded scrape pads on them after they got ground through. They are hanging right off the headers, and some little 2 1/4" scrap pipe out the sides in front of the rear wheels. sounds awesome, does not rob power.

On the other hand.. I tried a dual inlet/ dual outlet flowmaster in stock position behind the gas tank, it robbed power. It's difficult to speculate if the power loss was due to the muffler itself or the long exhaust run out the back. Sounded great, though.