Butch, no easy answers, but I certainly like your open minded approach and transparent thinking.
In my eyes, having such a big deal race where one group has two distinct classes with the same lap records is problematic. Nobody wants to get hung up racing an out of class car while the car in your class eases away for a trophy.
While I see downsides, I'd seriously consider moving the SSMs to the GP group. The cornering speeds are pretty good on SMs, which means that the ITR and ITS cars won't get killed/held up badly thru the esses, and the power differences are such that passes should be easy. And I would ABSOLUTELY do a split start with them, but not one of those halfway back split starts. Keep them as tight as reasonable to give the everyone a chance to spread out before the lapping begins. To me, that makes more sense, and is the least damaging way of achieving your goal. (strong entries, reasonable fees)
Now, that creates issues with turnaround for the drivers, so I'd flip the group order, and run group 2 third, and group 3 second. That would have the side benefit of making the IT guys happier, as they can watch the other IT races. (It's all about us, Butch!
)
I think it's important to keep in mind the cornerstone of your event, which is to provide the highest level
class races.... and adding a class with the same lapping times to an existing class is going to destroy that cornerstone.