Are you trying to restore the car back to factory appearance or simply making the car as original as reasonble possible without being 100% purist about it?

I see no reason not to do as you're proposing. It will look nice if done well. The spare tire tub is easily replaceable if necessary since it's not a structural part of the car.

About the only way someone would know if you had the Line-X done would be if the car was judged by Corvette people in a show. In most multi-make car show no one would likely know the difference anyway.

If you would ever decide to sell the car, simply be up front about the change so as not to misrepresent it. Since it's a replaceable part it shouldn't make any difference in selling value to most. Some potential owners may look at it as a plus...something already done and not needing attention.

I'm trying to get the car back to factory but I know all aspects can't be done with the budget I have. Any and everything I do will be handled as best as possible (with the 100% purist in mind).

My '69 tub was in pretty bad shape too - had been scrapped to the point it had an open gap. I bought a small fiberglass repair kit and used that to patch it. I was able to cut the glass fiber so that it conformed to the transition from the bottom to the side of the tub, and after sanding it down and painting it, you couldn't even tell it was repaired - unless of course you are an NCRS Master!