I helped my friend do this swap in his 4dr a little under a year ago. I've heard people talking before about having to cut the firewall but as far as i know that's more for a CRX. The only real cutting you'll have to do is for the shiftlinkage and the passenger side (tranny) mount bracket. Here are some sites that may help you out with some more info:
http://h22hybrid.20m.com (Pics and lots of info)
http://www.h22a.org/ (Very helpful people on the forum)
And of course it'd definently be a good idea to contact
S13GTR since he's done the swap and knows what he's doing.