Dont waste your money on "high flow" cats their about the same as stock ones.

 

first things first SEARCH is your friend top right corner, next get the trans build with a new converter2800rpms,and then posi, you allready have 3.73 gears in the tahoe maybe 4.10s so gears are good, next boltons then cam it. getting to 400hp n/a is very hard on the 5.3, good luck.