I think you're '79 should already have the better dual outlet manifold, so you have a good start already.. THe first step is go bigger! For an N/A 2" pipe should be adequate.. One of my Rabbits has 2" and one of the others has 2.5 and I really notice no difference.. I have a good ole glass pack on my DD.. I like the sound but it is LOUD... Some variety of "turbo" muffler seems to be the ideal, good sound but not crazy loud..
As far as an off the shelf exhaust system there doen;t seem to be much available.. Best bet is to seek out an exhaust shop, or break out the mig and get some pre bent angles...