Lots of questions here
I'll try my best.
1. My guess is that install a roll bar should be easy with the interior gutted... but I dont' know for sure.
2. Not sure how much you can get for the lights, but I would suggest that you just keep them. Correct Marchal lights would cost much more, I think.
3. Yes, you can get springs rearched. I'm not sure how cost effective it is though. New springs can be had for $150 (Dallas Mustang ad, page 73 of the latest issue of Mustang Monthly. I don't know how much re-arching costs.
4. If you get a decent paint job for about $2000-$3000 and do all of the other work yourself, you should come in at or under $5k.
5. The Green Hornet was a '68 coupe and therefore looks like a '68 Shelby. Lil Red was a '67 coupe and looks like a '67 Shelby. That's the difference.
6. I don't know how much your car's worth, but I would try some of the 390 sites!
Have fun personalizing her!
-Winston