Spindles are different in '71 and later, though I don't know that '70 and back are all the same (to '63). If you are thinking about disk brakes you'd be better off grabbing a '71 or later front (to '87 on pickups, '91 Suburbans and blazers).
I've seen the online description of narrowing the frame you mention (the HAMB?), or at least a similar one, but I've not seen anyone notch the frame.
You're cutting out the part of the frame that gives it strength so adding that strength back in and doing a good job on the welding will be important. Also given that the places the crossmember bolts to the frame are exactly the same places you are modifying you'll need to provide for suspension mounting.
I'd like to see pictures as your proceed. This should be an interesting project.
