Intersting question. I know the inner and outer rear fenders are slightly wider, (for a drag car that's a good thing especially if you plan to min-tub)

I would imagine you could graft them in for the rear 1/4's and the tail panel. (you would have to change that too) then I would think it would be easier to replace the front fenders and you would have to also graft on a partial door skin for each side (easier than trying to get 69 doors onto a 67).

Do you plan on keeping the 67 front bumper & grills etc? The front Valance would have to be modified as well as the leading edges of the fenders to make them work with a full bumper... all of this would take some fabrication, but I don't see why it wouldn't be possible. It all depends on how much of the work you can do yourself as to how feasable it might be budget wise & time wise... Or maybe you just restyle the wheel well openings by grafting in some of the 69 body lines and feather them in?

At that point a 69 shell may be a bettwr option?