OK, Pual Turner Mids (bottom end pipes), a wood port, 4 mil stroker (and the various bits it requires), cool head, single 35mm carb, and as much compression as the gas you are willing to buy can handle. This will pretty much solve the problem, but then you will lack top-end power.
If you let off the gas going up hill on a shee it is goin to fall of the pipe. You could try modulating the clutch and throttle instead of just droppin the clutch and floorin it, but that'll smoke every clutch you put in in about 6 months.
You could also try passing the 450's in the open sections of trails, then you don't have to worry about hittin 'em in the tighter sections.
Its not a jetting issue IMO, it is just falling off the pipe.