I'm assuming you are having trouble getting the high arch rear spring to sit level. If you look at a Model A rear crossmember (I know it's not a Model A spring) you will see that the crossmember cradles the spring over a distance of 5-6". You should have better luck if you make a mount that conforms to the spring arch and have the u-bolts 6" apart.
I've had a little trouble with stock Model A springs in a stock Model A crossmember. Had to make sure to draw each u-bolt up evenly, a little tightening on one, then the other. Back and forth.