Here's a system that I had on a 68 Cuda with Hooker headers that I later replaced the headers with stock exhaust manifolds after the pipes got crushed too bad...
Here's another system that I had built for a stock 70 Dart Swinger with a 318 and stock manifolds. Notice how we put a split in the h-pipe to make it easier to r&r the trans when needed, or else put the h-pipe further back behind the tans tail...
All of these cars were plumbed by Watson Engineering downriver Detroit, MI. The guy who did the custom work retired and they no longer do exhaust work for private individuals. They also made custom prototype exhaust for the auto manufacturers...
Find a qualified shop to do the work... It doesn't sound like your guy is not at the proper skill level for this kind of work...