Sandy,
My dad bought me a copy of this about the time it was published.
http://www.ebay.com/itm/UNDERSTANDIN...item27c7fb4225
It exhaustively covers the ARC-5, with articles culled from QST magazine and/or the ARRL handbooks of the era, including at least one power supply. You can start modifying right away from the articles. These have a "smoke 'em if you got 'em" attitude about power, it's happy with a wide range of DC supplied. There are also editions of
Surplus Radio Conversion Manual by Everson and Beach who cover it from a different workbench.
Now for the big question: how badly molested is it?