Hi Gordon - welcome to Nitroexpress.com forum.

For that ball, I would go to Jeff Tanner in the UK for a mould.
I now have a bunch of his moulds, from .595" to .740" and every one is very nice.

A .790" is .016" larger than 10 bore, btw. A true 12 is .729" to .730".

I personally would slug or measure the bore and groove diameter best I could and get a mould about .010" to .015" smaller than the land to land measurement.

http://www.jt-bullet-moulds.co.uk/