A 3" collector would hurt low end torque though, unless your running a big cam and heads. Might want to take that into consideration.

 

Does anybody know what size (collector and primary) OBX headers really are?

 

I think consensus is that the collector is 2.5", but the primaries are questionable. Maybe ask the guy that's selling them on ebay if it's a typo or not?

 

OBX headers are 1.75" primaries with 2.5" collectors. The headers are the same for 4.8/5.3/6.0 kits, but the 6.0 kits come without a y-pipe since the SS/Hummer exhaust is dual in/out muffler and the 4.8/5.3 has a y-pipe for the single in/out muffler.