303british.com -- Mr. R has a web site focused on the .303 and Imp 303 and with link to the Epps gun shop still operated by the offsprings of Elwood Epps. Mr. Epps did conversions from .22/303 to .338/303, both factory case necked up or down and "improved" case with shoulder enlarged for more powder capacity. Loads for P14 can be much more "robust" than for the SMLE rifles. Reportedly, factory .303 case can be loaded to equal .308 WCF easily. BUT NOT in SMLE rifles... NOT!
Mr. Ackley, US gunsmith, did a 2 volume set of books on cartridges from his multi decade career and many known fellow gunsmiths,-- factory, Improved, and wildcat. I think the bulk of the Epps rounds are in vol 2. AND Mr. Epps also did a conversion of the p14 to ".303 Canadian magnum." He used the .300 H&H mag case necked up for .303 bullets (from the British--.311 actual diameter, I think) and enlarged it to have a shoulder, not unlike the .300 weatherby Mag for .308 bullets in the M17 Enfield (US .30/'06 version of the P14 Enfield). Books are available thru eabco.com, sinclair International, ebay sometimes... (search P.O. Ackley and ignore the nurses stuff. guess there was a nurse named "ackley"...)
Since gunsmithing is basically "machinist" work, I suspect you would be best served by buying the reamer and having your work done locally. No idea what the laws are like in SA. In USA, a gunsmith is a machinist with an FFL (Federal Firearms License). Your bucks. luck