The DC8-63 wasn't a stretch, it was a re-engined DC8-62, I think you're thinking of the DC8-72 & DC8-73 which were stretches of the latter
Wrong way round I think.
There were 3 60 series DC8 variants. Firstly the 61, fully stretched but with same power/aerodynamics as 50 series. Effectively high density/medium range. Series 62 was only mildly stretched but had power/wing enhancements for ultra long range. The 63 married full stretch with new wing - high capacity AND long range. All used P&W first generation by pass jets.
The 71/72/73 were 61/62/63 re-engined with GE CF56 high by pass turbofans.