[Rebel1]

It doesn't matter what age your body is Boxhead...any glassing you do now is new glassing so it will pull panels if you are not careful. If that join is not structural then the flex filler would be best.

Yep, you can reshape fiberglass with heat. Within limits of course. Essentially you are slightly "softening" the resin. The straight edge is to hold the sill straight whilst it cools to the new shape. You may not get it exact...but close enough to not have to use a $hitload of filler before ya paint.

I'd use a piece of timber as a straight edge as it wont cause a hot spot. I'd also wedge the sill a little beyond straight because it will spring back a little.

I had the exact problem on both my sills. They are now straight after 3 heat cycles.