The Kaplan pumps do taper to a smaller diameter towards the head. This was not a problem for me since when I measured it a long time ago I think it was a 1/4” difference. To me that didnt mean anything, it thats a big deal for you then you should definately go with some other brand. I have been using it since May. I dont know if in the grand scheme of things if it would matter to me. There have been guys around here that have told us they paid as little as $60 for a handpump with guage and a cylinder. I think I would have paid about $90 but whatever works for you. There have also been guys that have paid $400 for an electric pump set up. There are plenty of pricepoints to pick from and a number of reputable pumps around.
Good luck.