To my knowledge you cant do that.
just change the cromwells gun to be exactly the same stats of the IST?
2nd one can. Go to the hardpoints, find a line along the lines of "weapon", find the IST's also. Take the IST's weapon line and copy it into the croms one.
ive donre this but afaics theres no ist gun in the harpoints and when i copy the weapon details into the croms slot it ends up with no main gun