Not Looking Like a Factory P/S Hose Arrangement

Considering where you are and the availability of parts, I'd be temped to just braze a 5/8" fitting in place of the 3/8" one and call it a day, assuming the pump, brackets, and pulley are correct and working properly. You may have to remove the reservoir to do it, so, try to get a reseal kit for the pump, or at least an "O" ring for the reservoir.