soft brakes and headaches

-The slant has plenty of vacuum
-The pushrod has to be set just right; there is a procedure.If it is too long the compensating port may not be opening, in which case it would be impossible to bleed. If it is too short you may not be getting boost.If in between, but not right, then you could be getting differing results. So, yes, its important.
-Just because your stock pedal bolted up to the booster pushrod doesnt preclude that its right.
-You state that the caliper pistons are about 1inch; do you mean wheel cylinders at the rear? If yes than that could be the problem. The 1 inch bore requires a lot of fluid to move through to activate them. A lot of fluid means a lot of pedal travel. The front pistons are practically touching the discs and so require just a tiny bit of pedal travel to activate. So you begin to see the delemma.
-I think you should properly adjust the rears and try again. If still same, then remove just one drum and have a buddy apply the brakes a bit at a time while you are observing the wheelcylinders for action.If no action then problem may be air. If some action you will need to make a judgement call;ie is it enough to energize the shoes.A smaller wheelcylinder will provide more action for same amount of pedal travel.
-An easy way to tell if brake shoes are near proper adjustment is to apply the parking brake.If it requires a lot of lever travel then they need it.
-Also, while the rear wheels are in the air, you can do a sortof stall test. Apply the brakes (with drums and wheels installed). Then with engine running and trans in manual low,give it some throttle and see how much brake effort is required to prevent wheel rotation. If it really requires "full on",then something is definitely not right. Watch out for stones flying off the tires.
-Im still thinking the problem is mechanical in nature