There have been a few such projects over the years. One small quibble is that a Lee-Enfield bolthead is nominally 0.585" and the .45-70 rim diameter is 0.608".

Another interesting cartridge for the adventurous is the .444 Marlin which has a rim and case diameter much closer to .303 dimensions.

None of these cartridges feeds well out of an unmodified .303 mag, though. The easiest fix is to solder a couple of internal ribs inside the rear of the .303 mag. box to produce a single column mag. of sorts. And then you will just about be able to stuff five of those fat rounds in. Then again, if your target "game" hasn't expired after 5 well-placed .444s, you are probably in the wrong place with nowhere near enough rifle.

On a historical note: remember that the progenitor of the family, the Remington-Lee, was chambered in .45-70 and similar-sized cartridges like .43 Spanish.