I don't see a problem with it. The fact that all the weight of the car is supported by the U-bolts and threads isn't an issue in my opinion. If you do the math, U-bolts that size are unbelievably strong. Besides, most automotive rear axles were done that way for years - and we made it worse by adding lowering blocks.

My only suggestion would be to put a plate on top of the axle to spread out the clamping load across the whole spring pad, rather than just on the "ears."

I'm wondering why the suggestions to weld the plate to the axle. I see no strength advantage there. The plate is simply a pad or spacer. Besides, I wouldn't want to apply heat to the axle and take a chance on messing up the grain structure and weakening the axle.