I've replaced the very stiff mainspring on three different Uberti '75's, with good luck on two of them. The third '75 ended up with a trigger pull that I considered unsafe for me, 'cause when I tried CAS style rapid shooting with it I had several premature hammer drops resulting in a quicker bang than I liked. My solution was to put the original spring back in and trade it back to the store from whence it had come for something else. I assume though that a good 'smith could correct the too light trigger problem.
By the way, I shoot one handed.