As you've noticed by now, drill bits usually don't drill round holes, no matter how careful you try to be. I'd suggest going at the guide from the bottom side where it's not battered yet and using a piloted chucking reamer, not a drill bit, to cut the guide to within a few thousandths of the OD. Better idea- - - -spend a few bucks with a machine shop to have them set the head up in a Bridgeport mill so there's no wobble whatsoever and plunge cut the remainder of the guide out that way, possibly with a 4 flute end mill. At this point, continuing to use a drill press will probably guarantee a ruined cylinder head.
I sort of recall recommending an AIR hammer, not pounding on the guide with a regular hand held one. Here's an example of a driver for guides that fits an air chisel:
www.ebay.com/itm/284394251543?I usually buy straight shank chisels and turn the pilot in the lathe to whatever diameter I need for the guide I'm driving.
Yes, a piloted guide driver is the only way to get one to move without chipping or otherwise damaging the old guide or the cylinder head. At this point, the prudent thing to do will be to bite the bullet and turn the job over to a machine shop with the equipment that might make it possible to save the head. They might have to ream the hole a little bit oversize and custom fit a guide to the hole with the proper press fit.
