911rs, I can't speak to the 350 but I did put Headman Headers on my 454. I also put on Flowmaster 70's for mufflers.
I had lots of room and did not replace the AIR tubes from the pump.
Check out the folks at Summit Racing - High Performance Car and Truck Parts | 800-230-3030
and give them a call. They know their stuff and the prices are great.
Go ceramic coating too.
BTW you've got a rare bird there. Most of the coaches had 454 but the very few as you've found out.
1984 28' Funeral Coach
Former Rolling Showroom & PuttLab (now party bus)
"I'm not an expert. But I did sleep in an Airstream last night."