Since you are using mostly Duff stuff I can tell you my real world experince. Mine was a 91 Ex using the 5.5" EB springs and Duff buckets.
I got aboout 7"-8" of lift and it rode excellent. I tried the 3.5" springs but they were way too soft for a daily driver.
There is a guy around here that has your exact same setup (B2 w/ 302) and he is running the 3.5" springs and says it is fine.
Either way, I'd lift it and ditch the body lift. You can run 35s with either setup, just depends on how much you want to trim.
If you copied that info from their site, it's wrong. The arms are .313 wall. You would need the kit for the C-channel frame. The one you have listed is for the EB w/ boxed frame.