Take another look at the drawing, there are thumb screws (not labeled) there to "clamp" the firewall in place. The whole idea is to have a test stand that will work for basically ANY motor. Yes, as you have stated, you will have to provide a mount for any of the beam mount type of engines, but you are going to have to have the mount to bolt the engine into the airplane, so it really is not an issue. You are right that it might be a little bit faster to get you engine mounted up with the PSP stand, but I suppose that that is only an issue if you really must have that engine running right now. Otherwise the difference is only a couple of minutes.