Years ago this topic came up frequently. The 303 Epps has a capacity less than the 7.62X54R that it resembles.
Unless the guy really wants a "303 Epps" and the custom dies and fireforming, IMO he'd be better off rechambering with a common 300 Win Mag reamer and creating a "303 Win Mag".
It used to be common too that folks would rechamber a "P17" to 300 Win Mag and employ a P14 bolt in it - such as I did.
Given how inexpensive new rifles now are, the ultimate cost of sporterizing such as suggested makes no economic sense, it's just a "because I wanted to" affair.
All that aside, in a P14, the 303 Brit can be loaded to 308 Win MV's and a "303 Epps" would add not much more than 100 fps to that.