When installing the hammer are you pulling the trigger all the way back and holding it while you drop in the hammer? You then install the hammer in a cocked position being sure the sear portion of the trigger is between the part of the hammer where the cocking notch is and the spring loaded lever in the front of the hammer (It's called the sear).
If this doesn't work there should be no problem caused by removing material from the back of the hand, but that shouldn't be necessary. After all, S&W got it together with the parts as they are so it has to be technique.
FWIW, the hammer can be installed first , then the trigger and rebound slide. but it is a pain because you have to hold the hammer slightly to the rear while trying to install the rebound slide. I have done it out of pure laziness when fitting the cylinder stop, but it is hard to explain just how it is done, but easy to demonstrate. Maybe this is how the factory did it when repairing your gun.