I am running 19-39 on my 700 SLP single pipe, union bay porting, cheater head, with a 162 PC I think it is about perfect I think you want to be around 80 MPH top speed for mountain riding. the 19-39 with 7t 3" puts you at 79.4MPH.
The stock gearing of 19-39 with 2.52" drivers top speed is 85.8 MPH at 8200 rpm
just by going to the 3" pitch drivers you have already geared down about 5 MPH which is just about right for any 700 or 800
keep in mind that most of these grearing combos you will have to change to a 74 pitch chain
if you go to a bigger bottom gear you have to move up to a 41 which will drop you down another 5MPH which over all i think is a bit too low.
here is the gearing break down
with 7 tooth drivers 3" pitch at 8200 RPM
19-39 2.05 ratio 72 pt chain top speed 79.4 MPH
19-40 not recomended
19-41 2.16 ratio 74 pt chain top speed 75.6 MPH
19-42 2.21 ratio 74 pt chain top speed 73.8 MPH
20-41 2.05 ratio 74 pt chain top speed 79.6 MPH
21-39 NR
21-40 1.90 ratio 74 pt chain top speed 85.6 MPH
21-41 1.95 ratio 74 pt chain top speed 83.5 MPH
my union bay 900 with a 153" pc I have 40-22 which put me at 89.6 MPH about 10 MPH faster than my 700 with 162 but it is a shorter track with 200+ horse power.