Ok, so here's my question: will Arrow slip-on exhaust from a '03 620 fit onto a '01 900?      will I need a cutting torch, BFH and a hard hat?

Thanks

I checked the sponsor Motowheels site and they have Arrow slip-ons that fit Monsters from 620-900-1000.

Cool, but could the same set conceivably fit all 3 bikes?  or at least be moved from a '03  620 to a '01  900???

Yes according to the website. There is common exhaust header size between some models and adjustability of exhaust brackets I guess. Take a look at the Motowheels site.

Same cans will fit all M400/600/620/695/750/800/900/S4/1000.

That's pretty clear and definitive. Thank you!