That's the setup I have: LTs, cats, single-in/single-out 3" Magnaflow with a single 3" exit pipe. I'm another one who thinks you'll 'feel' the loss of low-end. The 1 3/4" headers on mine are too big for my setup, IMO.

 

Yea im not going to run any cats on mine. so im looking for the biggest magnaflow i can find.

 

Dual 2.5's is over kill for that 5.3.....Anything more than a single 3'' is a waste of bottom end on a 400Hp small block. Build to compliment Torque and HP will follow, this is a heavy truck.... Every 1/10th you gain in your 60' is worth 2/10th's at the end of the 1/4...I can promise you that dual 2.5 are not going to help out your 60' times....

Dual 2.25's or a single 3'' would make your truck a pleasure to drive around town and a winner at the track!

 

Thanks I will definitly be doing a 3" single. I just dont want it to be loud.

 

Omitting the cats is gonna make it loud. I don't know if the big Magnaflow can quiet it down enough for you but hopefully it will.