I have a 2007 4.8l Silverado and I was thinking of putting speed engineering 1-3/4 long tube headers and I was wondering if can I weld a flange on it and bolt them up to the oem cats ?

 

nope, but you can rework the factory midpipe to maintain cats and attach to the long tubes

but the long tubes will occupy the same space your cats currently do. No plug and play solution there. Welding and cutting required. Or just get the catted y pipe

 

Speed engineering doesn’t offer a catted y-pipe last I checked.