Michael – I have a 3.5 inch on mine and find it "just so" but f I were you I would carry on with this monster and see how you get on. My guess is that before very long you will find that the height of your new vice will start to cause problems even with the mill head at its highest setting.
The positive side of course is that if your vice is sold on the auction site it would – with luck – get you enough cash to pay for a new three or four inch from say Axminster or similar. I only mention Axminster because I bought a 5" from them for my shaper a few months back when they were at an "offer" price and find it to be surprisingly accurate.
Rik