If you compare the .303 Brit case to the .30/40 Krag case you will fine about $0.10 worth of difference. In Canada, Mr. Epps, for one, did alot of modifying the .303 case (and rifles therefore) with factory taper and with an "improved shape" to calibers between .22 and .35, I think, .33 for sure. I can never remember the gun shop address, but it is on a link from .303british.com by mr. Redgwell. Obviously, the Smelly loads cannot equal the P14 loads, but respectable to say the least. Mr. R covers both on his web site.
Mr. Ackley included this information in his 2 volume set of books available from eabco.com, Sinclair, ebay sometimes, ... Mr. Epps is mainly in vol. 2.
In the US the widely available case (for single shots especially) was the .30/40 Krag.. Mil surplus. CHEEP!. It, too, has been taken eveywhere. Mr. A wrote that the .30/40 Improved (in an appropriate rifle, NOT a Krag, P14 for instance) would equal FACTORY .300 H&H loads. (The H&H were not loaded hot for use in places expected to be HOT, like Africa, India, etc...) And the .25 Krag full length (he also shortened it like the .219 Don Wasp on the .219 Zipper) was the same capacity as the .257 Remington Roberts Ackley Improved. Supposedly the best all around cartridge for the .25 bore... .25/303 Imp would be about equal. Might want to hold back pressure in a Smelly.
And this is a bit sturdier case than the .45/70. The .40/65 is on the .45/70 rim as is the .38/56 (not .38/55 on .30/30 case). .38/56 Improved is quite a thumper. .348 is on the .50/70 rim, slightly larger. A well respected "thumper" from the Yukon/Alaska area and for lever action pressures that should be no problem for the Smelly. Feeding? Don't know. .348 was expanded to .40, .41, .458 and .50 and called "Alaskan." Wildcats, so pressure up to loader. luck.