Somewhere on the web is a spring chart in .png format listing lengths and center pin locations, I'm at work without much time to search, I think it's on the keylime site along with the flat top knuckle pic's.
http://littlekeylime.com/MrN/mrnimages/ ... ngs_sm.png
Found it.
From what I recall the GM 56's may move your axle forward a bit.
People have used that lower hole, like Stuka mentioned you will need to plate the inside of the frame as the slots don't perfectly match the location of the outer hole, welding a nut to the plate is a good plan. You'll need to remove part of the lower box to keep the spring from hitting at droop like Stuka mentioned also. IIRC you will also have to add a few washers on the outsde frame area to match the factory positioning, I would tack those in place also.
Since you are trying to keep the budget down you can remove the rear most mount from the frame and flip it over, it will have to come forward a bit to keep a good shackle angle.
This does not exactly match the front SOA for height but since you are playing with rear springs anyway here's what I would try.
Use the FSJ rear main leafs to keep the axle location correct and mix and match the rear spring packs to get the desired height. Don't weld anything solid until you have everything where you want it.
I have moved all my hangers around both front and rear for axle location, just use some really good tack welds, keep the rig partially supported except when testing with full weight and be really careful.