As Fox said, there is a hole in the frame that you can put a long drift through and tap the pins out from the backside. At least my '50 3600 had the holes there... and my spacers were also on the outside when I took the rear springs off. A bench grinder is my tool of choice for removing U-bolts. Yours sound like toast anyway, if you had that much trouble getting the nuts off.

~ Dave
1950 Chevrolet 3600 3/4-ton with 261 engine & T5 Transmission