If you have duals I would not change till you need to. The HI-PO exhaust is simply a 2 1/4 pipe all the way back with 2 staight through mufflers, one before the axle and one behind it. It should be relatively easy for a custom shop to make up.
The Hi-Po exhaust is a larger diameter crossover into a 2 1/2" intermediate pipe. The muffler is a low restriction Hemi style muffler with a 2 1/4" over axle pipe into a straight through resonator with a chrome (or stainless) tip. I got mine from Accurate LTD. It fit perfect and looks and sounds great and is as correct as you can get unless you have NOS parts. toolmanmike