I use perimeter bolts with my ATR headers without a problem, but no heater box. I moved all my A/C stuff behind the dash with a vintage air kit. Plenty of room for the 3.5 downpipe in the engine bay now. Removing the downpipe takes three minutes giving me perfect access to header bolts and especially spark plugs. The bigest pain I had is aligning the front brace for the turbo since it is one piece with the header flange.
Greg Kring