Bill,

  Take a small torch, and heat the block around the oil fill tube, and work on the tube with a twisiting motion with a pair of channel locks, or vise grips. (takes a bit of heat before it will move, but once it does, don't stop moving it. You can also take a hammer and bump the vise grips from the bottom as you twist.) It takes a while to get the block hot enough, but I really don't think I got it hot enough to do any damage. Didn't even get hot enough to discolor the powder coating. But it does require a bit of patience. To install a twist on cap, I took the fill tube off of a 12a, and cut off just the thread and flared portion, and TIG welded on a short piece of 1 inch EMT electrical conduit. (about two inches, yours may vary) and with just a bit of sanding around the conduit, it comes to a press fit into the hole left in the block. My plan from there (which I haven't gotten around to) is once it is pressed (read 'driven in with a rubber mallet') I am going to put a bit of JB weld in the little grove left around the joint. (you will see what I mean once you get the old one out)  That method worked well for me.

Ben Schneider RV7  Renesis nearing 1st engine start
Advice worth exactly what was paid for it.