This picture shows the area above the transfer case cross-member on the passenger side of my '75 GMC that makes it hard to fit an exhaust pipe. This truck has a TH350 and NP203 transfer case.
The downpipe for my Flotech exhaust might have worked better if it angled a little closer to the floorboard, but then there would have been more heat into the cab. I was able to get it all bolted in place and keep it about an inch away from the crossmember and the transfer case. I believe they design this system with a two part header pipe (you can see the clamp) so that you can rotate the 2nd pipe section and get the angles you need to drop down after you go past the transfer case.
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Most of these trucks originally had single exhaust and the single pipe was routed on the driver's side where there is way more space above the crossmember.
Bruce