Jump over to the Big Bore Forum and click on "510 Wells Build Questions".

Then, in the same forum, click on "XAUSA And His 505 Short Range Express".

Most of your questions will be answered.

Myself and others here have built rifles on the 1917 Enfields. The Eddystone is least desirable. I prefer the Remington actions.

Any way you do it, using the Enfield as a base for a custom rifle is difficult if you have no serious metal working experience. Expensive if you hire someone to do it. It is cheaper to use a CZ550 or Brno602. It is real easy to make a ugly Enfield conversion, takes a lot of work and $$ to make a good looking one.

I am not familar with the 333 case dimensions and overall length, it may be too big for a 1903. If the COL is not too long, my first choice would be a 98 Mauser of recent origin, maybe a Mark X.