depends on what car looks nicer and what is rusty beyond repair. Of course the rusty one is the race car, and both would make nice criuzers. I would have no problem hacking up the chevelle,lol, but the cougar would be lighter for a race car. So both have there pro's. Do you want to cut the cougar, or cut the chevelle, do you want to drive the cougar everyday or chevelle. Chev will be cheaper for racing, we all know that, so it's a matter of preference actually. Good luck deciding.
Well heres my thought on this the obvious is make the racer with the one needing the most work to be streetable,but I would also say the cougar would be a nice cruiser and there are less of them on the road then chevelles.