Sounds like you are gonna need to pull the cam. Had a bunch of slants with varnish in the lifter bores to the point the lifters wouldn't come out the top. They would push up a bit and then stick. If this sounds familiar here's what you do. Push the lifters as far up as they will go and then pull the cam. The lifters will push out the bottom fairly easily. Since you are planning to keep the cam, make sure you keep the lifters in the same order they came out so you can reuse them. Putting new lifters on an old cam is a good way to wipe a lobe and ruin an engine.