If you remove the slide and carefully drop the hammer, the hammer strut will be in it's most relaxed state and about 0 tension on the main spring.
Wrap something around your grip safety to keep it engaged and tight up against the sear spring, so that the sear spring doesn't pop out of alignment.
Push main spring housing pin out from left to right with soft metal punch, remove existing main spring housing, make sure that the sear spring tab is in the frame slot and slip the new main spring housing in while guiding the hammer strut straight on top of main spring cap, install main spring housing pin from left to right and your done.
shoot safe, shoot straight, and have fun
good luck
PS. before anything you might consider getting a Smith & Alexander MSH with mag-well, come in arched or flat.