Not trying to talk you out of the Montana Rifleman Action but
I would start with a Remingtion action for your first biuld.
The reason is that they are very simple to blueprint,they respond
well to accurizing and a good donor rifle is inexpensive.
Also good Stocks are easy to find for the Remingtion.
Plus as a gunsmith you will probably build more custom rifles
Remingtion actions than anything else.
Plus after your finished you will have a nice custom rifle
Just my opinion
J E CUSTOM