Starmac, before you bolt anything down think of where your powder measures will be. Do you mount them deeper, and reach, or beside your press, or are they mounted on the press? Which measures are used with which press? Are your presses dedicated for pistol and rifle or are they used for both? Do your loading blocks go in front of the press (I have a Brown Bair that mounts on top of the bench) or do they go beside the press? Lots of details to think about in the actual use that go well beyond the physical space for the press. Clamp a press or two to the bench and actually load some before you start drilling holes.
I load sitting in an old office chair on casters that swivels. This impacts how I mount things.