It depends on how patient and mechanical you are.
Fitting a safety on a BHP isn't that hard, assuming you are mechanically inclined enough so that detail stripping your HP isn't scary.
If detail stripping your HP is not something you feel comfortable with, then send it to a smith.
Otherwise, it is just a matter of using a small hobby file and carefully filing away the section of the safety that engages the sear. If you file away too much, the safety won't properly block the movement of the sear, and you'll have an unsafe gun. You have to remove a little bit, reassemble the safety/hammer/sear assembly and try it, remove a little more, and try again, etc...
There were some good hints on how to change the mainspring on
www.fnhipower.com, they'll come in handy.
It isn't brain surgery, but if you aren't good with delicate, patient work, then you'll ruin a $40 part, or worse, have a defective safety.
If you get the C&S extended safety, be aware that it has some unpleasantly sharp edges - you'll probably want to "dehorn" it and reblue.
Steve