Generally the best method is taking some chevy 454 or 350 headers and a set of pontiac flanges 3/8 to 1/2" thick.
Cut off the chevy flanges heat up and slightly bend the chevy exhaust so it fits within the pontiac flange. Then weld it up or tack it to a point someone else can weld it.
Custom headers often for much less then pontiac custom headers will cost you. That and you know they will fit.
3/8"-1/2" header flanges will run you around $50-100. A second hand 350 or 454 headers will run you around the same $50+ depending on condition etc.
Jumping on a pontiac forum with your make, model, year, engine, etc will have you coming across what works to give you the fenderwall headers.
If none of those come up. Then take measurements of your space/restricted areas, maybe make a rough thing out of cardboard and throw it in the trunk before heading to the swap meet or meeting someone on marketplace etc.