To answer your question. The replacement/after market hemi h-pipes are the same part number for 1966-70 Dodge or Plymouth. The muffler angles are not different for the 66/67 applications versus the 68-70 applications. The tailpipes are the difference for 66/67 applications as the fuel tank and gas filler tube are designed differently than the later models. In other words later 68-70 tailpipes will not fit 66-67 applications properly. As far as wheel bases are concerned, there are not two different h-pipes to accommodate the two wheel bases as there are not any fit issues for installing the h-pipe on either one.
Hope this information helps.