what unit are you installing?
both youngrodder and solidaxle are running the gen II compac in their ad trucks
parkwood's tf has the 55-59 surefit system.
i wanted the newer electronic controls and ran the gen II compac in my 58
i highly recommend that you install the unit before you paint your cab
this way you can weld in brackets and captive nuts if needed
the downfall of this is that your unit will be out of warranty by the time you use the ac
but they are very forgiving over at vintage air, i needed parts and help 7 yrs after buying the unit
va was very accommodating
