so I am wanting to get some rubble out of my subie and have come to to the conclusion that pouring 100's of dollars into headers, high flow cats, etc. really isnt worth the minimal power gains, so i want just weld on a muffler. Im leaning torwards flowmaster because the sound and it doesnt look all ricer-ed out

but I' too busy to get under and measure the diameter I would need.
I am always reading youre guys forums for any questions i have so, what are my best options?
Thanks!!!