It’s a Canton Racing Products 2 qt. Tank # 80-201, powder coated satin black.
Yes, battery is in the trunk. Coolant overflow tank and electric vacuum pump for the brake booster reside where the battery used to be.
I’ve owned the car since ‘79. That hole in the fender brace has always been there. I thought GM installed it, guess not. Lol.

I think the system is sound. Excellent point about boiling point. I hadn’t thought of that. I’ll try a new, lower pressure cap and see what happens.