Well, I love my single actions, but have a similar problem- the trigger guard does a job on my finger as well. I have really big hands, as you might also. Some folks do not have a problem at all, just depends I guess. There was probably a reason the previous owner had Pachmayr's on that gun.... A stock with a filler behind the trigger guard protects the finger, at the expense of slightly lowering your grip on the gun. One method of grip that seems to help is to wrap your little finger under the butt of the gun - this lowers your grip slightly, and gives you more space between the trigger guard and your finger.
Usually, if I am going to be firing really heavy loads, I use one of my Smith 29's, or my Redhawk, as rubber grips are an eyesore on a single action (just opinion). If you cant find a hold that works for you with the wood grips, try the pachmayers. If they protect your finger, then you might want to consider having a set of nice wood custom grips made that follow the Pachmayer pattern with a filler behind the trigger guard, only a little slimmer in the rest of the grip, without those finger groves.
Larry