John- You should try running some Rislone in the oil for a while first. That stuff is amazing when it comes to freeing up sticky lifters. If you don't want to do that, then I would suggest getting some Kroil. This is a super penetrating oil that comes in an aerosol can with a stem. You should be able to shoot some down the pushrod hole. Then, I have found it is also easier to remove lifters from a hot engine. If you can, idle the engine up to operating temp and then quickly pop the valve cover off and go to work. The hot block will allow a slightly enlarged lifter bore. You will still need to use the lifter tool, which was a very wise investment, but it should come out easier. I would also suggest replacing all the lifters. If you have one bad one, others will probably follow.
Hope that helps.