its not that the bolts will move persay, its a metal fateage thing, take a coat hanger and band it back and forth a bunch of times, it will be ok for the first little while, but eventually it will break in half. same sort of idea, the studs will flex, so will the edge of the pipe that the stud goes through.
i built my first sas kicks (my wifes and mine) rear shock mounts like you did, as well as 2 friends of mine. all 4 trucks broke within the first 5 trips or so down the logging roads hahahaha.
oh, and watch for cracking at the bottom of the extention where you welded your spring hanger on. the hanger may want to rip off over time.
-Lance