The best option is the Cromwell if the late war Ammo in the 6lber is effective. I read the 6lber even with the mid war ammo could kill a tiger at close range through the frontal armour. I don't think the 75mm could do that at any range.
32 Mph with a low perk and with an effective ap round would be a good addition mayne even 40MPH if we had the unfgoverned engine.
At 1000 yards the 6 Pdr APDS Shot Mk.I (Armor Piercing Discarding Sabot) could defeat 110mm of 30° armor. So, it would have a fighting chance but it would have to use its speed to the utmost because it doesn't have the armor to slug it out with anything. The Cromwell's armor was on par with the Sherman and Panzer IV. With the 6Pdr and the LW high velocity AP rounds, it might just add in something we do not currently have: a fast (tank), medium armor, fast firing (20 shots a minute), good AP ability under 1000 yards, average AP ability under 1600, and worthless AP beyond 2000 yards. Oh, and a poor HE round (similar to US M4/76mm).