I'm doing a No.4 in .45-70 right now. I started with one of the prechambered, prethreaded $85 barrels Sarco was selling a couple of years ago. The barrel isn't cut for the extractor, so it can be fitted to either the No.4 or No.3 receivers.

You have to shave a little off the "thumbnail cuts" inside the receiver that guide the cartridge. The SMLE receiver has a web the barrel butts up against, like a 98 Mauser. The hole in the web won't pass the larger .45-70 rim, and has to be opened up.

The standard Enfield extractor and ejector work without modification. The SMLE hook has a lot of travel, and pushes the rim against the left side of the receiver, where it contacts the ejector screw as the bolt as pulled back.

There's a semicircular cutout on the right side of the receiver ring, to allow for the point of the original 210gr round nose .303 bullet to be ejected. Ejection of a fired .45-70 case works fine, but a loaded cartridge has the nose of the bullet hit the receiver ring and it will jam up most of the time. If you pull the bolt back slowly and pick the unfired cartridge out with your fingers it's OK. You could open the notch in the ring up without affecting strength much. I haven't decided what to do yet.

I had the loan of one of the Gibbs .45-70 conversions for a while, and compared it to my parts closely. The Gibbs magazines are available from Old Western Scrounger, and while they'll hold four cartridges, they'll only successfully feed three. Looking at the magazine and cycling cartridges through the action, it looks like you could adjust the feed lips and follower to make things work if you took the time.

I don't see any reason why the .45-90 wouldn't work. I would have gone that route, except the barrel was already chambered in .45-70, and I had brass on hand.