When things are stuck, I usually heat them up. If you don't have a torch, you could always stick the engine in the oven at 300 degrees for about an hour. That gets all the parts warmed up to 300. I used an infrared thermometer on my engine when I put it together, using the oven method to install my crank bearings (NEVER BEAT ON THEM). It took about an hour to get the engine to 300 degrees. Make sure you use welding gloves, because three hundred degrees will peel your skin off!
A note on torches, you'll never get it off with a little propane torch, you need an Oxy Acetylene torch to really heat that bugger up.