DoubleRifle999,
If you have already decided to use one of the guns you already have, I would tell you to look at the stocks on them first. I would be surprized if most of these older gun stocks would hold up to much shooting of the 470 cartridge,so that means a re-stock job. Which of them might be easier to restock? Next, I'd look to see that both of them are "On Face" and don't need to be re-jointed, that can be a rather daunting job to do before you even get strated on your conversion.Don't even think about converting a gun that doesn't lock up super tight. Look at the breech faces and see if one or both of them have bushed firing pins, if so, that is a positive. I also like vented breech faces but you probably won't find that on either of your actions. Look to see which one of them locks up the tightest, after a good cleaning of both of them, including inside the recesses for the greener crossbolts, that will give you a good indication which one was made best and/or shot the least.

If you like a cheekpiece on your gun you can give extra credit to the gun that already has one, as well as the sling swivels being present. I agree that your barrels should be 24 to 26 inch long, that will give you enough length to burn all your powder but is short enough to be handy to hunt with. If you plan to restock it, do you have a stock duplicator or access to one? If not you may want to start looking around for that service and see what that is going to cost, as well as a good strong stock blank.

Save your original stock to use as a pattern. I regularly build up the originals with auto body putty to get a higher comb or to add a cheekpiece, plus I put some on the wrist area to thicken it up a bit to make it stronger. Then go over that filler with a thin coating of Brownells Acraglass to harden the surface a bit. You can even create or remove some castoff using this method as well as building places up that need a little more strength. A good hint is to leave your pattern stock slightly larger than the finshed stock will be, that will give you a little room to fudge it one way or the other without running out of wood as you fit it to the action. It is much easier to work the last bit down by hand than to slightly over cut the new stock and have to start over again! Don't ask me how I know that!

On most of the conversions I have done, the problem was how to get it as light as possible, but with the 470 you may want to start considering how much you want it to weigh when finished and plan your barrel weights accordingly. I would err on the side of being just a little too heavy before I lightened up the barrels too much. Ask people who have one or have built one for their barrel profile recommendations, weigh all your necessary parts and balance the gun accordingly.
Check all the gun auctions for any 470's and average their weights to get your weight corrected. Good Luck with the new project gun. Bob