todaRS200

I currently have a B16A tranny out of a 99 Si and I am dying to go B20 with my car, but I'm afraid that freeway driving will kill my motor. So what I need to know is how difficult is it to swap in an LS 5th gear into my tranny? Also where can I obtain the parts needed? If from a dealer, anyone have part numbers? If anybody can give me some info on how to do this swap, or direct me to a site covering it, I'd greatly appreciate it.