Well I'm probably passing the point of no return now

I don't hate you for cutting an original car that belongs to you and only you. My 74 Duster is an original 360 car that they only made 12-1300 or so of. I like it better the way it is. I don't blame you for doing this.
How big a tire are you running now? I'm running ladder bars and leaf springs with floaters and am running 14x31 Goodyears with the spring relocation kit with stock frame rails and mini tubs. I have a friend with the identical set-up(suspension and tires) and he has run a best of 9.30 @ 145mph... with a Jericho 4-speed. The ladder bars were a big help. The 4-link will be good but may take a while to sort out the set-up. There are a lot of adjustments to be made and also a lot of ways to screw up.
Happy cutting and good-luck.