Vortec..... I modified our frame and it worked out great for us. I'm with Red that the bolt on is nice, quick and easy. Money was the issue on this build and it was cheaper for me to mod the frame than to ditch a brand new compressor we already had to buy a Sanden and a bracket for it. One thing I noticed is after the modification the p-side frame is the same width as the d-side. GM only made the frame fat there so they had somewhere to mount the idler arm. That's my Good luck with your project.