kombi: I just went to a link that published 303 Epps data. They had one caveat - don't use that loading data in the SMLE actions - apparently the round is higher enough pressured that it sounds suitable for the P-14 action. I believe that if you loaded the Epps round to 303 Brit pressures you would be alright in the SMLE but you would lose the advantage of the Epps design.
To be honest, I really like the old 303 and do not feel it gives up anything to the 308 (for comparative purposes only). With that in mind I doubt I would go for a P-14 in the Epps design - you wouldn't get that much more out of the 303 caliber, from my perspective, to justify either the expense of the conversion or the increase in weight by going from the SMLE to the P-14 action.
When I reload for my 303s, I reload to original factory specs (mil-spec, actually) using either Vitha-Vhoury Oy (? spelling) or Norma N203 powder. Both list charges for original factory loads. I would rather go that route than go with a conversion but, that's just my opinion. HTH. Mikey.