It's a pretty simple design. Neck the .303 up to take a .375 bullet - you might have to use a couple different sized neck expanders, but it is a simple procedure. A .375 decapping pin/expander in a Hornady die set will do that job in one pass.

Seat a .375" bullet and mic the outside of the case neck with the bullet seated - add .003" to .004".

Use SAAMI or CIP print-out for all the other measurements - but suggest keeping them on the tight side - or actually measure your brass and add a thousandths (.001") of an inch.

I would personally go for an improved case, but simply for shooting cast bullets, the standard will work just fine, I'm sure.