A buddy of mine has 2.5" Flowmaster Delta 40 series muffs on his 67 Pickup w/ a 400sb. WOW>...the best sounding muffler out there. However the magnaflows will flow better..(just by lookin at them). However I have a 64 Impala w/ a 454. I just put on a full 3" system w/ delta 40s. Starting it tomorrow..Should be NICE