Since you are into the Creedmoor for a bunch of $ and have made the chamber cast, you might as well have RMC make you some proper sized cartridge cases.  Time to learn about paper patched bullets.

How about posting some photos of your rifle, please.

Just a note here. I'd never depend on dial calipers for determining bullet size, or groove diameter. Need a micrometer to accurately determine such things in my opinion.

fogman,
congratulations on the interesting journey you are about to undertake.
you will find it most interesting, and will unlock much of historical interest if you go the full mile.
it is now known that later in the era, long range rifles were often, though not necessarily, breech seated.
this was found to do the same thing as muzzle loading, but easier. harry pope went through the same process in scheutzen.
those rifles were designed for bore diameter pp bullets, and were the peak of that technology. as such, they work best with them.
they will also work well with fixed ammo.
there is a discussion on the Shiloh forum about breech seating creedmoor rifles, which also goes into internal dimensions of Remington 44 barrels.
it appears that 44 is a fairly flexible term, with many of them being what we might now think of as tight 45 cals.
it is just a matter of using reasonably thin paper and a correctly sized bullet.
I envy you the opportunity you are embracing.
keep safe,
bruce.

All - thanks for the encouragement - I will keep you posted with pics and range results on this original. I have been reading the Shiloh 44-77 thread too. I have some experience with my black powder Shiloh Sharps and I really enjoy using black powder. I just started with local silhouette match shooting which I am addicted to after only twice. So I am looking forward to seeing what this original is capable of in the way they were designed to be used. Breech seating, paper patch -fine; I think a complete original Creedmoor in this good of condition deserves to be shot as it was designed to be shot. Y'all don't go away - I will probably have some questions as I get into this project.

Do not use .458 bullets. The bore diameter you have is correct; this is .451 at the bottom of the lands. 
 
There is an old Lyman mold, 451114, made for this cartridge. The original loading was paper-patch and this was the offering for a lubed groove bullet. The mold was offered in various lengths for weight, I have the mold for 480 grains which I have used to paper-patch for .45-70.

You might also look at heavier revolver bullets, 300 grain and up, for the .454 Casull and .460 S&W pistol cartridges. This choice is if you want to only fire it at short range and not compete.

(I could be persuaded to part with this mold as the neurosurgeon has limited my shooting due to recoil after extensive neck surgery. He even offered to give my Sharps a good home at his cabin in Alaska.)

Holy criminy!  That rifle is at spec for the period and for PPB!  Back then the caliber was still measured by British Standards.  Look at the old catalogs....the mention bore!  So if my own 44/77 measures 451 groove and .445ish bore I'm just as happy as can be.  These rifles were designed to shoot PPB....not greasers and they shot them well.  I bet this one will too if you'll feed it the right groceries.

In my travels and adventures I have found that in Creedmore Ballards and Remington rolling blocks .451 is the correct size and your rifle bore was not fooled with. 
You can get a mold at .451 for that muzzle loading rifle Gibbs or something that is currently being made. Maybe Dixie or somebody like that. Cheap experiment.
   Don