My thoughts on the lightened flywheels: Panther, and all big British singles and 360* crankshaft twins, used heavy flywheels because you can't balance the motors. They were usually only balanced to 50 to 65% of upper end reciprocating weight, so they wouldn't vibrate fore and aft so badly. By lightning up the flywheels, and increasing the rpms, you may find significant vibration. I'm using a 250cc crankshaft (lighter weight, shorter stroke) in my B50 BSA (500cc), and I got blisters on my hands from trying to keep the handlebars from jumping off the bike. Heavier flywheels should not hurt performance if you have a long enough approach as we do at Bonneville. Drag racing is a different story, and lighter flywheels are usually beneficial for quick acceleration. If you have spares, you may want to try one before you lighten up both motors.
Tom