Since the 2500 HD springs have two more leaves (and a fatter and longer overloader), I couldn't use the stock size u-bolts. I also couldn't use the 2500HD u-bolts, because the thread doesn't extend far enough up the shank, and because the 1500 bottom plate holes are too small.
The solution was to have custom u-bolts bent up at Carrier Suspension in Abbotsford, for $10 each. I think that's fair for Grade 8 u-bolts with fine thread, deep well nuts and hardened washers.
Here's a comparison of the 1500, custom and 2500HD u-bolts:
U-Bolts.jpg