You need u-bolts to fit the springs, but they won't fit the perch if its wider, so the u-bolts will need to fit the perch. Now you need spacers to keep the springs inline with the u-bolts. So yes, you can add spacers on the sides of the u-bolts. I would use steel with bends on each end so they can't slide out or move around, or as Jerry said above, two pieces of angle iron across the tops, facing down the side part way, just be sure to ease the sharp corner of the angle iron so the u-bolts fit down tight.
Have you checked with Detroit Spring or your local trailer shop for the correct width perches? It's not hard to open up the diameter to fit a bigger axle tube if you could find a 2" width the right length. It has to be welded to the axle tube anyway, so being a little jagged won't hurt. Be sure to replace the u-bolts.
Does the '54 have 2" wide spring hangers?
You should have total weight on the rear axle to set the pinion angle before welding. Most guys just clamp the perches to the housings and finish the build before any welding, you then set the correct pinion angle, tack weld the perches, pull it all apart and do the final welding. Weld a little on each perch so as to not warp the axle tube with too much heat. Jump across and weld a little then jump back to the other side.
On my truck, the axle in it was already set right, so simply measured the perch angle compared to the pinion seal flange area, then transferred that to the new housing.
