Agree on the 2.5". I think 420-450 hp is roughly the line for moving up to 3". I have an XE-274 with AFR heads and it is right around 500hp in my 383. You should be in the 400-420 range fairly easy although your smaller carb may or may not short you a few HP.
I went with spintech mufflers just for something different. They are a little more expensive than flowmasters though.
Just noticed you mentioned spintechs above. Mine drone right at 1500 and really scream at 2500 when accelerating. At idle they are nice and throaty. WOT they are borderline obnoxious, to evryone but me
On the highway mine isn't too loud, and doesn't drone really. Cruising between 2200 and 2800 rpm you notice it but it doesnt bother me.
I have 3" super sport 9000's, which are about the smallest they make and dumping out before the axle.
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