Holy cow, you get into 5th and 6th gear on the track? Ive never once in my life hit 5th gear on any motocross track on my banshee, old 250R or 450.
Blasters must be geared tight in the tranny. Your initial gearing estimate may be an OK place to start, at 14/38. But, even with your motor work that much of a gearing change is going to take a toll on your power. I would definitely ride it with your current gearing so you know exactly how much the motor work helped, then go from there. Every time you go to the track I would take several sprockets so you can change them out after practice if you need to.
One thing is for sure. Riding a track from 2nd-6th gear slows you down a LOT. You need to cut out 6th gear for sure, and preferrably 5th also. But like I said, blasters must be geared pretty tight if you are starting in 2nd and making it to 6th before the first turn so you might not be able to eliminate 6th AND 5th.