Steve, I would lie if I'd say I have 100% accurate information but I try my best:
launch is rumored to be in spring 2003, first cars should make it to the US/Canada market around summer/autumn 2003.
Yes, the engine block is based on the GT1. It is actually a completely overworked "old" GT3 engine block with various enhancements. Power is rumored to be 380 HP.
The weight of the new GT3 is rumored to be around 30-50 kg
over the weight of the "old" one. Actually not very good news.
Yes, the GT3 will be available as a Clubsport version right away but as far as I understood, the only difference will be the roll cage and some minor track adaptations. No single mass flywheel as far as I heard but this info can be wrong.
No sequential shifting system as some rumors indicated but a highly improved gearbox with an oil cooler.
I hope this helps. As soon as we have more precise information, we'll post it.
A little hint: if you want to get a GT3, you have to put yourself on a list somewhere as soon as possible. In Germany, some dealers already have substantial order figures and I'm sure the waiting list in the US/Canada won't stand behind.