Check out the following thread for more options. Keith K and GoEflight did exactly what you are proposing. I took a slightly different route and used a 14 turn 010 with the Astro 3.3:1 gearbox.
With an APC 9x6" prop on 8 720's it had slightly less power than the stock 300/B. On 10 700's it has slight more power. Probably 9 cells would be about equal but I never tried 9. It flies even better with a 10x7 but it's too much for the NiMH cells, and it might be too many amps with 10 Nicads.
For stock type flying the Astro works fine, if you already have one try it out. It's easy to mount in the stock gearbox. I believe both Keith and Perry used the GWS pinion from the 300 drilled out to fit on the Astro shaft.
However the Mega's do look like a better alternative, if you are going to buy a new motor. Shame not to use the Astro if you already have it on hand.