As far as a custom built tank goes...........you probably could/should be able to find one that will fit. It may not be the exact size, but reasonably close enough that you could use a stock size available. A local fellow redid his Sunshine 36 from that mid 80 era, and he did not comment about not being able to find a new tank. Not sure if it was slightly smaller, bigger equal but different demensions......
Not sure what to say on the refer unit, as my Arcadia of the same vintage is only an ice box. In the sink area would be about the only place on my boat to put it!
She drives me boat,
I drives me dinghy!