The atlas, benchmaster, diamond and smaller burke / us machine co mills are too small for your purpose, as is the standard sized barker PM. At a minimum, you’ll need a sheldon, hardinge TM/UM, or the large Barker Mill. These will limit you to one fixture on the table at a time, but will get the job done well with a few workarounds.
With the larger sheldons, a nichols or a clausing horizontal 8540, and some creative fixturing (often drilling and tapping the table) you can shoehorn two fixtures in there. These only have a single t-slot, hence drilling and tapping. Nichols did make one with a larger table and a few slots, but kind of rare. Also a lot of nichols mills are 30 taper, though they did make some smaller amount of 40 taper.
Since you are looking for a dedicated machine, you don’t need a full toolroom model with micrometer handwheels on every axis. Your head/spindle will be pinned to one location (tube centerline), and your table X will be pinned to one location (center point of your cutting fixture), so all you really need is movement in the Y. Handwheel is nice, pneumatic/hydro/power is best, and even a lever can work well.
Good luck in your search, check out HGR just around the lake from you. They have several horizontals which will suit your needs right now.