Well, I think you have to start with some restriction, or hard point. It could be the rear gear, or the max rpm? Whatever it is that is going to limit you. Then the other choices fall into line. IF you say 6500 max rpm, then you could pick the cam, then you could pick the gear. Or if you pick a gear first, then you could pick the cam, and the rpm would fall into place. What is your limiting factor?
What Brent said about your current combo makes sense, too. Maybe you should see where you are at with your improvements before you make a fairly big change.
That said, I think solid flat tappets are the bomb for economical power and rpm.
My current solid flat tappet setup has about 150 lbs closed and 405 lbs open pressure and I think that is more than average. I would say average for a solid flat tappet in a street/strip setup is around 135-140 closed and 360-380 open. I have the Compcams Max Area lobes with 279 on the intake and 287 on the exhaust. I also have titanium retainers and stiff pushrods. No hint of distress to 7000 rpm. I think when you get up into the 425+ lbs open range with a flat tappet cam you have to be real careful about getting all the right components to survive.
JMO,
paulie