318 Exhaust

It should be fine as long as your down pipes aren't all rusted out and won't last as long as the rest of your new exhaust system. Gonna flow a lot better than a single exhaust and sound better too. Hopefully they will give you a little break on price for not doing the job completely. I had a dual exhaust done on a 72 Dart with 2 1/4" all the way to the exhaust manifold flange (so I know it is possible) but since the manifold outlet is 1.75" I gained next to nothing. Good luck!