When I put IPD headers on my 72 145, the only "adjustment" I made was to grind away a bit of the inner fender flange at the outboard extent of the header pipes. Without this relief, they wouldn't go onto the head and would have rattled against the fender. Other than that, the installation was straightforward. The car breathed noticeably better at high rpms, but I made the change because my cast manifold was cracked for the second time.