I am going to replace my front entry door with a fiberglass pre hung unit. Can I tap-con/anchor the new new unit directly to the block? The existing door, from the 1950's, has 2x6 and the door is directly hinged to that, so I don't think leaving the 2x6's in place would give me a large enough opening. Thanks!

Yes tapconning into block is fine. Try to stick to the mortor joints.