You know, if I were going to try to keep the rifle 'as is' and still lengthen the barrel, I believe I would try a method I did for a ballistic lab some time ago.

This is all predicated on the fact that the barrel is a 'heavy barrel'.

I would cut off the barrel just in front of the forend tip so that the forend would still fit correctly.

I would counterbore the remaining 'stub' to accept a custom liner.

I would purchase an octagon barrel of the same diameter and turn one end to fit the counterbore with a 'step' in it.  On the 'step' I would cut external threads to a 'machinist's fit'.   

I would cut internal threads in the 'stub' to accept the external theads of the new barrel.  I would screw it in with some sort of glass or epoxy compound along the unthreaded portion of the new barrel liner portion and seat it with a 'compression seat'. (about .005")  This means that the two shoulders have to be cut carefully so that the 'flats' line up.

I would drawfile the new barrel to match the profile of the 'stub'.

The breech end of the barrel must be 'faced off' level with the barrel 'stub' and provision must be made to ensure that the extractor still operates correctly.

The new chamber is cut and you have the option of any .45 caliber cartridge including the .45-70 Govt... unless, of course, the outside of the 'stub' is stamped with the original cartridge designation.

There is another way to lengthen the barrel while retaining all of the old but it is a little more expensive.