You'd have to look at the pressure limits for the Lee-Enfield's action
In other words, what is the pressure cap on the .303 British cartridge? As long as your .45-70 loads don't exceed those of the .303 British, you should be fine. But it is a rear-locking lug bolt action, so when you get up there close to loads meant for the Ruger No.1 and Siamese Mauser, I'd be darned careful...
