[classicauto]

A shop up in Canada here (rotary dynamics) gets alot of their hardware cad plated.

That might be an option to consider as well as zinc.

The cad'd bolts I've used from them have stood up very well, with nuts/bolts on the waterpump/housing also.

As for actual sizes, depending on EXACTLY what you want to change, most of the bolts and nuts are pretty standard metric threads, with the exception of a few bolts that are odd lengths or odd shoulders.