Ok first you need to know the engine setup.
Iron 5.3, Patriot stage 2 59cc 243's. EPS 222/226 .596/.598. Stock 78mm truck intake. 130ish shot on a plate.

Now my planned setup, I want something I can do once. Something I can grow into when and if I go blown or 400+ CI, but not be way overkill for the 5.3. (

The headers I have are LT's with 1 7/8 primaries and 3" collectors. I'm keeping them.

I was going to do a 3" y-pipe with Magnaflow "Hi-flow" cats, (gotta have them) into a flowmaster collector (3" -> 3.5") then 3.5" into a Spintech and exiting in front of the tire.

I keep going back and forth between 3.5" and 4" in my head. I think 4" would almost be too much, but it is only half an inch.

 

I think 4" is too much on a NA 5.3 even with all your mods. 3.5" should be enough flow for plenty of HP even with FI. I would probably go 3.5" and have a cut out put in right at the base of the collector to uncork it a little if you feel like you need more flow down the road. To give you an idea the 78mm intake is about 3" while the 90mm intakes are about 3.5".

 

I wouldn't go any bigger than 3.5in piping. For the 5.3L even with heads/cam and a shot 3in piping will do just fine. Hell i think 3.5 would be a bit much. Make sure you run a straight through style muffler to reduce restrictions.
Later on down the road if you decide to up the pill or go FI 3.5 will still work with either setup. 4in is way overkill IMO.

Here ya go, hopefully this helps a bit.
https://www.performancetrucks.net/fo...-setup-473593/

By the way, hows the V8 world treatin ya?! lol

 

Looks like 3.5 it is. But since I'm stubborn I'll go up to 4" just before it exits. Just for looks. No tip, just raw stainless.