The clearance between the headers and firewall looks OK to me, but do the headers line up to the cut out in the body? That would be my biggest concern...there are ways to realign the side pipe mounts, but cutting (or worse patching) the fiberglass is a lot harder.
For the side pipe mounts, you could use a rubber (or silicon) mount at the frame mount and weld the mounting bracket to the existing bracket to your pipes. (See link for example)
http://www.summitracing.com/parts/stw-uh3-5w/overview/
Another option would be to cut out the restrictive mufflers from the side pipes and replace them with more free flowing ones. This would give you the chance to adjust the overall length of the side pipes.
Classic Chambered Exhaust Inc. - Products