A lot of where you locate the new front cross-member (where to cut the floor, etc.) depends on the length of your 4-link bars. If they are blanks and to be cut and welded, you have some flexibility, but he rear has to be located pretty closely to where the existing rear is or you'll never get your tires in the middle of the well. Look in my album (68NovaSS, around page 7), there are quite a few pictures. There is quite a bit of measuring and setup to do before you start gutting the rear section.
Additionally, I have the spec and instructions sheets from my Alston kit and will send you a copy if you'd like, pretty much tells it all and I'd be happy to talk you through any problems you have when you do it. I also have the tub installation instructions if you'd like.
PM me or drop me an email if you're interested. I also have a .mil email account.
Good Luck, Larry