Gibbs rifles has Enfields in 45/70.
Don't know what model they are made from but I'm sure it's not a P-14 or a Model 17.
The only person I know that has one is not very impressed with it though.
Gibbs are butchers, pure and simple, IMO. Their rifles are Lee Enfields, not the Pattern 14 rifles. I wouldn't touch a complete P-14 rifle, it is worth far more than any .45/70 conversion would likely be, but actions are available if you look. I believe SARCO has some advertised, but they are not cheap! If you don't insist on building the rifle, you might look for one of the 1960s converted Siamese Mausers in .45/70. Probably be a cheaper route. I see them at gunshows on occasion.