Our DT's have 19 off 1/4 bearings in the bottom and 22 off 3/16 bearings in the top, the XT250G the same isnt it?
http://www.cmsnl.com/yamaha-xt250g-dual-purpose-1980_model8766/partslist/
you will find it easier if you turn the gearbox output shaft by hand as you are attempting to change gear, if you think about it...this is what would be happening when the bike was driven.
The swing arm bolt will rarely come out without sawing through it each side. However It's worth supporting the frame by placing a stout tube around the head of the bolt and resting this up against something solid (a wall) first.