Does anyone make a relatively inexpensive bolt-on dual exhaust for a 78.I'm keeping the ram horns.

Mid America does. www.madvet.com. If your keeping the ram horns it'll be easy. I have a 1980 and I put on a true dual system last year. I also added headers, so I had to do some modifications. For your car they also have a chambered system similar to mid sixties sidepipes, only it's all under the car. Very cool!