rear shackle location

I'd say how the factory came up with the pinion angle is by building a model, working out the measurements it took to achieve what they wanted then made a set of blueprints and built all the cars to those measurements. Here's what I'd do if I were in your shoes. I know by experience that the trans. angle is generally around 0-1 degrees positive. You want the rear to be exactly opposite that under a load. The type of rear suspension, any traction aids, power level and driving style (cruising or racing), are all factors in figuring how much negative pinion angle you need. If you have stock leaf springs and it's a moderately to high powered vehicle that you run hard or race you generally shoot for 5-6 degrees negative angle so when it's under a load it's cancels out. If it's a stock 318 you don't need that much negative angle.

Are you sure your front spring mounts are for an A-body Barracuda? I have a 68 Cuda and they are not like that. Both are equal length and straight. If I remember right E-body Cuda's have angled springs. I don't know for sure if it's due to the frame design or the front spring brackets are different but if they are maybe someone has put E-body spring hangers on it.