FrankS:

If I was you, I think I might have a look at the .375 JDJ. Gives you the .375 bore, and dies and reamers are available, and the case will give you a little bit more capacity than the .303 and if you are going to handload you can tailor the loads to the action. With the greater case capacity you should be able to reach better velocities than the .303 case can produce, at less pressure. I do not have experience with wildcats in Enfield actions, but do have experience with the standard .303 in many Enfields, both No.1 and No.4 actions, and tho the action of the No.4 is strong, case life with full power loads can be quite short.

The .375 JDJ uses common .444 Marlin brass and should not require any bolt face work and probably will feed fine in your No.4 mags.

The .375 JDJ will give you essentially the same ballistics as a 9.5x56 Mannlicher or 9.3x57, which is a bit better than you can hope for in the smaller .303 Brit case. I think after you put together such a rifle you would change your mind about limiting yourself to cast bullets, as with jacketed bullets the round would make a super killer on all American game out to 200 yards or so.