Piston will be too big at 5/8". Never fussed on Willwood on motorcycles, HD used them at one time and they never got sizes matched so brakes were crap. (may have been a design feature?) Nothing wrong with cast master cylinders, just about every single one you'll find is a machined casting, including cars, trucks, buses, etc.. You were just unlucky with the one you had. Contrary to You Tube backyard casting video's, a lot can go wrong with castings if temperatures, quench, etc are 'wrong' Grain size, trapped gasses and strength are compromised