When I had AFE Power LT Headers with HF Cats installed by a local performance exhaust shop they had to fab/weld in a 2" long piece on each side to make a 3" performance connect work.
The AFE's came with a connector to the 2.5" mount point used by catbacks, but I wanted 3" the whole way. The 3" Borla LT Header extensions only work/fit if you are using Borla's catless headers.
So we were left with about a 2.5" inch long gap between the AFE and the Borla. The shop cut sections of the Borla 3" extensions and welded them in to close the gap and make the 3" performance connect. They did a nice job with it and its a clean looking job and all T304 stainless, not anything I could have done on my own though.
Pretty much any headers will connect since the 3" Borla still uses OEM connections up front. We have a pretty awesome deal if you call us @ 443-730-9428!
Do you want the system to be a bolt on affair or do you want to have a full 3" exhaust?
For a bolt on affair, pretty much any kit out there that is considered direct connect or factory connect will bolt up to the Borla cat-backs. American Racing, Kooks, Pypes, Stainless Works, Stainless Power, AFE Power, etc, we have them all. My recommendation to you would be American Racing, Kooks or Stainless Works as well as select others. If you are more looking for a certain price point, let us know and I'm sure we have something that will fit into your budget. Note: Most kits will be 1-7/8" primaries, but there are some 1-3/4" options available on our site.
If you want a full 3" system, the American Racing Bottle Neck Eliminator kit would be just what you are looking for. This system will eliminate the OEM 2-1/4" connection and cross over that is built into the cat-back and give you full 3" exhaust as well as a much more efficient ARH cross over (H or X). They do require cutting but instructions and hardware are included.
Hope that helps clear some things up, if you have any questions, please contact us.