From the info I found on this forum, Summit Racing 9040 (or 9140 for shiny option) fit well with a magnum. Also 680144 exhaust kit should work very well, just need to trim the pipes in the right places by about 3 inches.
Hoping to do this swap over the winter
Use search here or google to find topic called "Summit Headers and 2 1/2" dual exhaust in '68 A-body w 5.9 Magnum"
These are the ones I used with very little banging on them. The only place I had to bang on mine was at the power steering shaft. You will have to drop your steering linkage to get the headers in but not a big problem.
