I installed an aftermarket spring kit in an acquaintance's Ruger New Model 357 Blackhawk about 15 years ago. I don't recall the kit's manufacturer (was Wolff in business 15 years ago?).
Anyway, I don't believe it took more than about 45 minutes to an hour to install. As I remember, the owner was pleased with the results. If you have a bit of mechanical ability, a good set of "gun" screwdrivers and a little patience you shouldn't have any problems. Just read the kit instructions thoroughly before starting.
As an aside, on my own old model Single Six (69 vintage) I just clipped a couple of coils off the mainspring and it cleaned up the cocking action and trigger feel nice enough that I don't feel the need for any aftermarket spring kit in it. And it has always been 100% reliable, YMMV. If you decide to try clipping the mainspring, do it one coil at a time and check your work after each snip.