Frank, As an "oldtimer" I've had some experience with exhausts on 235s and I never had any problem with less back pressure from using duals and glass packs. They always ran better, but usually required a jet change to go with the improved ability to breathe.
I experimented with a cross pipe, the same size as the other pipes, but couldn't tell any difference one way, or another, except it did seem to make the exhaust a little quieter.
Any good welder can cut up your exhaust and re-weld it with an extra flange for another pipe. I use a nickle rod and pre-heat the cast iron, first. Makes a pretty decent exhaust system.
