Almost any year of mustang will outperform a boxster given some modifications.
A Boxster is a beautiful car, but has a very serious problem that can't be fixed.
You can't get to the engine to work on it. It would need a complete redesign,
so it is like a GT40.
For this reason, I think a Corvette is a better choice for an 'old' car that you will need to work on to keep working and tweaking.