Well, i'd say keep the most realible car in you're fleet. If the Regal has given you 0 problems and has overall been a good car, i'd keep it because finding a good solid car is hard to find. It sound like the GTPs history is unknown so keeping it is a gamble. Depends on how well it was taken care of before.
If your dead set on not having an L67 car in a year, why do you want to mod one now? Makes no sense to me. Think ahead, If you don't want an L67 car in a year, don't mod one now, save the money! Demod the Regal, clean up both cars, sell them both, and get your GTO. Simple as that. No wasting money in between...
