Maybe call Grizzly and tell them you have a G0705, and ask them how to adjust the bearings.
Basically, the spindle runs inside a tube with a tapered roller bearing at the top and the bottom. There should be a large nut above the top bearing that is used for adjusting preload. There might also be another large nut under the bottom bearing that is used to retain and protect the bottom bearing from debris. The nut will likely be a spanner nut that is best adjusted with a spanner wrench (not an open end wrench like the Brits call spanners). I really prefer the single pivot style spanners to the ones with a plate and movable tips. You can just make one from some steel scrap and some steel "piano" wire that you buy at a hobby store that caters to the RC crowd.
The tube section has the rack teeth that engage the downfeed for the quill. Remove the down feed shaft and return mechanism, and the tube should slide out. There might be a large snap ring that prevents it from falling out as well.
All tapered roller bearings should have their preload adjust while they are being rotated. It does not have to rotate very fast, it just has to be in motion. As for how tight, you will probably be fine if it is just tight enough to eliminate end play plus a tiny bit more to allow for expansion.