I don't use Saito engines, so I won't give any break-in details, but as far as the rest of your Q's:

that engine WILL be a little gutsy for that plane, just remember that the throttle has many positions, USE THEM! That plane will fly very nicely at 1/2 throttle, and 3/4 will let you do just about anything.

As far as moving the firewall back, Yes, once it is moved back, you can still put a .60 up front, you'll just have a little extra room. And as far as the mount goes, more than likely, you'll have to replace it. Now would be a good time to make sure that you can access the back of the firewall later to remove, and reinstall the blind nuts for the mount, because the new one (according to Murphy's Law) won't line up with the old holes.