I'll probably be dubbed a red neck, but I've always removed main bearings by heating the case around the bearing with a blow torch, then using a drift of some sort on the inner race of the old bearing to drive it out. They tend to come out quite easily.
Stick the new ones in the freezer the night before fitting them (this will shrink them) and heat the case around where the bearing will be going in (this will expand it). Now you need something BIG to sit on the outer race of the bearing so you can drive it in. I have a large socket (something like a 2") that I use. I just use this and a copper mallet to drive the bearing in. I've done quite a few sets like this. Just use some common sense with regards to heat and force, if you're swinging the hammer over your head it's too much force!