Varmit_Master,
I have the same gun and conversion you have except I have the 24" barrel. I shot loose Pyrodex and then swithced to FFFG 777. It gives more velocity and burns somewhat cleaner than the Pyrodex and Pyrodex Select, I had previously used. I used 120 gr. of powder and you can feel a difference in recoil with the 777. My 700 was not finicky and shot well with all these powders. The hotter the load the tighter the group. Never played with pellets.
I use BM3 in my Encore and it is a great powder for me. It was slightly more accurate than 777 but clean up is easier and I know I can shoot 6 times with out cleaning between shots maybe even more. I haven't shot the BM3 in the 700, though I feel it would perform just as good as the others.
Do some experimenting and see what 's out there. Wally World is discounting all there hunting stuff now and you may find a deal on 777. Black Mag 3 is about $30.00 per pound if you can find it. Maybe some of the other powders will work well but i haven't tried them.
Good Luck,
Stan