If rebarrelling, a .243 Enfield would be an interesting conversion but the bolt head would have to be modified for a rimless cartridge (that would also be true for the 7.7 & 7.65 cartridges mentioned above). Possibly a bolt head from one of the Indian No. 1 Mk III's in .308 could be adapted, but I don't know what all might be involved. Staying with a rimmed cartridge will be less problematic.
I'd have to check to be sure, but I think the only difference between the SMLE and the 2A bolthead is the steel used and the extractor. No design difference.
It couldn't be used in a No. 4, completely different designs.
ive seen one of those turned into a pistol. ive never seen it fired but i hear its frightening. its definately a one of a kind sort of thing. maybe you could do somthing like that.
I've heard of some Aussies doing that mod, usually it gets fired once and that cures whoever fires it of wanting to do it again!
Re: 45-70 recoil - if weight doesn't bother you, try putting lead in the butt trap. Should hold maybe a pound or so? Supposedly works great at reducing the felt recoil.