Wow, I'm seeing a lot of advice here that would kill someone if anything goes wrong.

I have a small milling machine... it weighs 2500 lbs. When I bought it, the riggers set it in my garage on 4x4s. I used various combinations of hydraulic jacks and wooden blocking to lower it to the ground an inch at a time, and it was TERRIFYING. I will never do anything like that again; one slip, and you're a nice soft pile of ground meat. I have since built a gantry crane that will easily lift 2 tons, and I use it more than I ever thought I would.

I've read about people building gantry cranes out of wood, and when used in the right way, it's plenty strong. We build houses out of wood, after all, and a waterbed can weigh 1500 lbs.

As to putting the mortiser on casters, now would definitely be the time to do it. Don't skimp on the casters, and don't trust the weight ratings. Expect to spend $100 for four quality casters that can hold that kind of weight, less if you get all-steel ones. I would build a base out of some sturdy angle iron and design it to get the machine as close to the ground as possible. Do you happen to know a good welder?