Front and rear springs are the same lenght and pin location.I ran stock 4 leaf springs when I just had the rear kit installed and the springs were way to soft. I didn't get any more lift then my stockers with a add-a-leaf. I ran the Rubicon and half way through the trail the rear was bouncing on the bumpstops and spring wrap was nasty(better shocks might have helped). I then installed the front kit and needed another set of YJs so I got some Pro Comp 2.5" lifted springs with the softest spring rate(4cyl,softtop) and threw them on....way to tall! so I disassembled the stockers and Pro Comps and mixed and matched springs until I had the ride height and stiffness I wanted. It took a couple trys but I have the rear packs with the top two Pro Comp leafs and two stock. The front is top two stock and one Pro Comp.
After another trip to the 'Con (with 33"s instead of 31" tires and a traction bar for the spring wrap problem) the flex was awsome but at the top of Cadillac hill I was on the bumpstops again(had a weeks worth of gear and a co-piolet onboard)but the front was perfect.
Sooooo I decided to get another set of 2.5" Pro Comp springs or something else and try again when I get done with the Toyota axle swap. I do run DT Prerunner shocks up front and Superlift softrides in the rear so shock performance might be a issue here and I'm going to swap them front to rear and see if that helps. One thing I highly recommend is a traction bar, the longer softer YJ springs warp up real easy even with my 4to1 TC and 3.73 axle ratio.
As for the approach angle compaired to stock, I don't know. I had a Calmini shackle reverse with stock springs SPOA and they hung down pretty far so the TT kit was way better. On the rear I do drag the longer shackles of the TT kit a little more then the Calmini shackles but I was also hitting the gas tank, bumper, and dragging the 33" spare hanging of the back. I think that was from taking harder lines and being more confident in my rig.
Thats where I'm at now, your 4.3 is alot heavier then my 1.3 you'll have to find what works best for you on the type of trails you run. We expect to see some wheelin' pics when your done too