A quick thought, i think the uk heads probably have slightly off centre lifters (to the lobe) to promote lifter spin and therefore equal wear across the lifter (i think! could be wrong), so the roller lifters you have would have to be of larger diameter than UK lifter allowing the follower bore (in the head) to be offset bored? meaning the roller tip would sit in the centre of the cam lobe?

Hope that makes sense???? Or have i just over complicated things.... and the machining is only to give a method to 'lock' the lifter into place and stop it from rotating.

Rob,