The 45/70 conversions does not require any modifications to the bolt or receiver rails. If you have a sporter then your good forthe stock and maybe scope mounting if drilled and tapped. You need to rebarrel, chamber and headspace. Forend needs to be openned up for the heavier 45-70 barrel. Install sights if you're going that way. The Gibbs mag has all the bugs worked out...you can use a military mag...lots of fiddling...I mean lots and I've never seen one work right.
There are very few folks who will work on an Enfield...John Rempel is the only guy out west that I know who does.
Good luck with the project,
Ron