BigBill: Yes! Gibbs and Company have converted the SMLE #4 Mk1 from the origianl 303 British Caliber to the 45-70. They seemed to have worked out the magazine feeding problems you would expect from such a conversion but I haven't heard anyone talk about theirs yet. There was even a fella in England who converted his old warhorse to a 45-90 (same pressure range).
Lots of folk like us think they would be great guns in the field and anywhere else but I don't know how many fellas have them or what they think of them or how they shoot.
The pressures of the 45-70 and the 303 British are just about the same and you can convert the #4 MK1 without running into pressure problems if you use the loading data for the 45-70 Government. Yep, those Brits are one smooth, quick handling action.
Now BigBill - since you have done your own rebarrelling, you could prolly convert a SMLE to 45-70. Of course, if you convert one to 444, you would have to use a 24-26 inch bbl with a 1:20 twist, and then I would hafta buy it from you, but only after a couple of samwichs, a good Bigfoot chase (tough to get them to pose for a family photo I hear - unless we get my seester involved) and a trip to Dudleytown............ With Mikey you get more in 2004. Mikey.