Well, I used to work as a professional mover, so I'll throw in my 0.02 for ya.
A fridge dolly is probably the best way to go, depending on how narrow the stairway is you wouldn't be able to get enough guys around it to hold it up, at least one or two on each end of the safe, depending on how big each of the guys is. With a proper fridge dolly you shouldn't need the 2x10, they have tracks on the back, and you can just pull it up using those one step at a time.
I'd say make sure you have plenty of beer iced down and pizza on the way for when its done too.
However, depending on where you are having it bought from, most places offer delivery services, and it really might be worth a few bucks to have it delivered as opposed to all the hassle and possible problems you could encounter doing it.
Aside from actually getting it up the stairs and set up you have to consider transpo too, I assume you will probably be using a pick up, which it could be a pain to get out of without damaging the truck or yourselves.
Just my thoughts on the issue, can you and 5 other guys get it done, sure, but its easier to pay the guys who do it for a living.