Body Filler for Inner Fenders

All metal I’ve worked with before. If done properly is some pretty good stuff. Very hard to sand once fully cured. I’ve actually used what I call a cheese grater used for plastic filler when it is like 70 percent cured to take down the high spots. It handles heat rather well but any place that gets exhaust got it will bubble and peel/crack.
After seeing you photo. I would not worry about heat there.