Thanks, but the Grade 3 bolts were not used as bolts, per se. They were cut down and used as locating pins only. They are 5/8" diameter, and fit tightly into mild steel spring perches, so I figured they would suffice as dowels. What served as "pins" in the OEM setup was the 1/4" spring-pack bolts, with the nut on the bottom of the spring pack fitting loosely into 5/8" dia holes in the spring perchs. Not exactly a precision arrangement, but no evidence of any movement or wear of the thin (3/16") spring perch material stock.
If the U-Bolt pressure is significant, the pins won't even be doing anything since the friction would hold the spacers in place as they appeared to do before the spacers (albeit under less stress).
Yes, I will be carrying tools along and will be re-checking the torque on the U-Bolt nuts frequently, especially at first, and looking for any bending stretching or distortion of any of the suspension components.
The U-Bolts are 1/2"X20 threads. I was unable to find heavier hardware locally, but perhaps I can find 9/16" U-Bolts and fabricate a top plate of 3/8" steel. Someone suggested that I should have used (solid) bar stock for the blocks, or could have poured the tubing I used solid by pouring them full of melted wheel-weights through the 5/8" holes, then re-drilling the holes 1/2" deep. Those are good suggestions.
I will take a good look at the spring keepers. To be honest, I didn't even consider them, but they serve a real purpose in multi-axle trailer suspensions.
As far as use, the trailer will see 95% straight highway miles with very little maneuvering. RVs live a pretty easy life compared to commercial trailers, but then they are built awfully light and shoddy, which negates the benefits of their light use.
Bottom line, I will monitor the suspension closely and upgrade the components per the suggestions here as I locate heavier duty parts.