An A=6 will work just fine.
When I got my 59 Fleetwood in '87, it was missing the compressor and the hot gas valve. Since this was pre=internet, I had no luck finding a replacement hot gas valve, and was told the A-5 was not rebuildable.
On the advice of a local Auto A/C shop, I converted my system to more modern cycling clutch system. I added an electric T=stat to the evaporator core and wired it into the clutch circuit. I had a welding shop Tig weld the hot gas lines shut.
I purchased a converted A-6 compressor from Vintage air (or maybe it was Old Air??). It had been modified to use the A-5 brackets. My brackets were missing, so I cobbled up something to work.
The system works fine. The only drawback is I am unable to adjust the evaporator temperature other than changing blower speed. In Yuma, Az, it wasn't a problem.