WILLY!!!!
Welcome to TEAM NOVA!!
As far as the tranny swap goes, the only thing you will need to modify is the tranny mount. You don't need to cut/modify the drive shaft since the 200-4R , powerglide, and the 350 share the same spline count. The drive shaft will sit in about an 1/8 to a 1/4 deeper into the 200-4R due to the tranny a little longer than the others. Other than that, you don't need to modify the floor in any way. You will need a 12 volt power source for the lock-up. If not, you will need to manually shift it in that position when you drive it.. Here is a good start for some info.
http://www.transmissioncenter.net/2004RTIP.htm
BTW, I have that same tranny in my 66 and if you need any help on it, don't hesitate to ask me or anybody on this site!!
