Quoted directly from Mustang Steve's web site, Hopefully he won't mind.
Although this kit fits the T-5 and Tremec factory cable type bellhousing, SOME HEADERS MAY INTERFERE WITH THE CABLE AT THE BELLHOUSING / CABLE AREA.
Tri-Y headers are especially problematic, but can be modified by relocating one of the tubes.
Hooker Super Comp long tubes fit well.
When using a 351W, the Heddman long tube headers fit like they were made specifically for this application, and their shorty headers fit great, too. Stock manifolds present no problems.