Why you want to spend the money on a M1917, versus just buying a $650 CZ550 in .416 Rigby is obviously a personal choice. However, here is what needs to be done. First make sure your remington is a sound action and not swiss cheezed with scope mount holes. Get yourself a P-14 bolt. Open up the boltface to .593 and deepen it about .003.The rim on a .416 Rigby is about .065. File the bolt rim till a .416 Rigby case will easily slip up under the extractor, don't cut into the boltface while doing this.. You'll also have to reshape the extractor somewhat. Buy a sheet metal mag box from Numrich arms that will fit the .416 Rigby. Fit it to a Flat triggerguard. Don't use that military one.
Now for the fun part you need to make a fixture to hold the action in the milling vise upside down. Now with a carbide 3/8 ball mill, open up the rear of the action and the bottom rails till a .416 Rigby will just drop in.You need to tilt the head about 8 degrees to do this. Machine the bottom of the action to fit the new mag box. You will probably also have to take out about .075 from the front. To do this tilt the action at about 45 degrees and cut a new ramp at the same time. Be carefull here ! You'll also have to mill back the bolt stop so the bolt will travel back further. This about covers how to enlarge the action without going into the fine details of exactly how it's done. Now with FILES only, you begin to open the rails and shape the ramp till a .416 Rigby will slide up into the chamber without binding. If you have even fair mechanical ability and DON'T use a DREMEL Tool, you will figure it out pretty fast. You will also need to make or buy a new follower that fits the .416 Rigby.
As for a barrel, get one from Pac-Nor and remember a 1917 Enfield uses Square threads. Next cone the barrel, chamber and headspace. Finally again using the mill, make the extractor cut. Of course you will now want to fit a cock on open kit since you cut into the spring recess while opening up the action. I also assume you've reshaped the action to get rid of the ears first. Oh yes you'll need a stock that has to be crossbolted and a recoil lug also needs to be fitted to the barrel. A .416 Rigby needs NECG open sight and a front barrel band ( all from NECG). All told, your looking at about $1000 if you do all the work yourself and probably $2500( and way up) if someone else does it.
That CZ550 sure looks attractive now! Given all this work and expense, why not build a real Big Bore wildact like a 470 MBOGO, 500 A2 a 585 NYATI or a .600 Overkill. The amount of work on the action won't be any more and you'll be able to easily justify the cost!
For what it's worth, I've done 5 Big Bore Enfield conversions. Never Again. It was fun learning how, but in the end it was not worth my time and a reworked Enfield is still a reworked Enfield with very little resale value. If you can afford it just bite the bullet and start with a CZ550 .416 Rigby action and barrel it yourself to a bigger caliber. By the way the 500 A2 conversion is a total snap!! Hope this helps-Rob