Will Prindle 18 sails work on a Prindle 19?

Anyone have any idea whether the P18 main will work on a P19 stick and provide adequate power?
Yes, it will.

No, it won't.

Luff is shorter by 2 feet, foot is shorter by 8 inches.
It will fit, have a big gap between foot and boom, and
have less power to properly drive a 19.