DO NOT stick flat spacers between the duals. They will cause everything to loosen up with driving. I tried it for a while. Pretty much had to re-torque rear lugs with every other fill up.
Adapters, which bolt down and have their own studs are OK as far as staying put. Just keep in mind that pushing the wheel out too far could put you past the 102" limit.
To run 235's on my rear axle (I run 265s up front) and maintain good spacing, I went with a deeper-dished outer wheel set, i.e. more backspacing (which may actually be more negative
backspacing on the outers.... I can never keep it straight....).
E- series dually rims are less dished than F-series. So just swappping to F-series dually wheels in the back should give you enough clearance for 235. However.... Les Schwab's used F-series inner steelies my last go around, and those caused rubbing between the tire and body when fully loaded and cornering hard. Had to switch back to E-series inner steelies (I broke both of my inner Alcoas so back to steelies for now).
My front rims and rear outer rims are matching 17" Ultra Goliaths. I actually went back to SRW front brake rotors so could run a wider 8" wide front rim, which is the recommended width for 265's.