With my Alpine's I was able to mount both the tweeters and mid-woofers in the same mounting positions as the stock speakers. I had to dremel a couple of shallow slots in the sides of the tweeters to fit the stock clips, but that was it. You cannot tell I have replaced the speakers - which is how I like it. It's does however seem a shame to throw away the cool Alpine adjustable tweeter mounts that came with the set

The bigger issue is wiring. Since I had non-Monsoon but added my own amp, all the speaker wires came from the head unit and I rewired everything with decent (not excessive) speaker wires back to the rear bulkhead where the new amp was (same position as Monsoon amp would've been). Most was easy, except it's quite a pain to run new (thick) speaker wires into the doors. The crossovers I velcro'd in up in the dash above and on the outside side of each footwell.
If you have the Monsoon amp, you could get away with re-using the factory wiring (though that's pretty nasty/thin stuff) and cut off - or adapt - the factory speaker connectors.