I apologize if this is a repeat question. I found answers pertaining to cassettes, but not cranks.

I'd like to gear up my Giant Rainier by giving it roadie gearing. Replacing the cranks (and cassette, but only if necessary). Having never torn a bike down that far, is it just a matter of adding the road cranks and obtaining a larger chain? I run out of gears going down hills and top out at about 35 mph. Is there an alternative I have overlooked? What is the biggest setup for MTB's? Thanks in advance.