I hope you're making a single shot 'cause that will be one SOB to make feed 100% reliably. If you are bent on this journey into the black hole of wildcatting for no particular gain, try Pacific Tool & Die for a reamer. The last 2 reamers I got from JGS had serious problems and the last one left me 1/2 way through a chambering job unable to complete it, with a brand new, first cut reamer. I will never buy another JGS reamer!!!
Some advice from an old wildcatter with several dozen designs to his credit............you play in the land of massive amounts of wasted money with no chance to recoup any, there is nothing you can think of, or do that has not already been done. Anything you may build or have built for you, will be unsaleable and worth the value of the action it's screwed onto. If you are not doing the work yourself the vast amount of wasted money just tripled. IF you do happen to come up with some fantastic new and innovative combo, someone else will claim it. Some wildcats are built to make use of gobs of available brass and some for greater velocity in a given caliber size and even others to use a case which is common and cheap and a given bullet which is also common and cheap. I can see no advantage , in your case, using a very obscure and virtually obsolete cartridge and then running the shoulder back 200 thou, to create a 7-08. Have you tried pushing back the 35 deg shoulder on the SAUM, it is not an easy thing to do and you will lose 50% plus, of your brass to collapsed cases and shoulders. To form these cases, you will have to make up a set of forming dies which will move the shoulder at the body back the 200 thou first, creating about a 15 deg shoulder THEN AND ONLY THEN you can work the neck junction back until you have your 35 deg shoulder. You will still lose some cases in the final step to collapsing shoulders. Now you have some formed cases, or do you? What you do have is some formed cases with a dual diameter neck, 1/2 your neck is the correct thickness and the lower half much, much thicker. These cases are not usable like this and must be inside neck reamed before you can seat a bullet and chamber a round to fire form to its final dimension, oh ya and they will also end up probably 40-50 thou short of original length. Now you can fireform, resize and trim them all to a uniform length and you're ready to start load development..........wrong......now you have to outside neck turn them all to reduce the imperfections and uneven brass neck thicknesses you created by inside neck reaming and then it would be a VERY good idea to anneal the necks and shoulders at this point so you get more than 2-3 loads before necks start splitting.
So you are willing to go to the expense and time investment to get to this point...........you have 50 or so cases made up and are ready to start load development.......what is your goal? A load that was easily doable with the 7 SAUM, 7-08, 7X57 or 7 WSM using roughly the same amount of powder...............WHY is the big question you have to ask yourself.
If you intend to start with an original 7SAUM are you sure the rate of twist will handle the 180 gn 7mm at these reduced velocities? Have you researched what rate of twist you will need for this cartridge, bullet and velocity combo? I just pulled a barrel after 40 rounds from brand new because I went with too fast a twist in a wildcat, it was a 300 dollar stainless steel Bevan King and I did the work myself, anyway off she comes and on goes another 350 dollar stainless barrel due to error in calculating rate of twist. It is impressive to watch a 90 gn 22 cal bullet turn into gray mist 30 mtrs out though!!
This dissertation is not meant to discourage you as much as it is to inform you of the pitfalls and unforeseen expenses involved in wildcatting. I've been there many times and I'm still throwing away money doing it, but I have gained a wealth of experience and knowledge in case design, what works and what doesn't, how expectations are seldom achieved and how there is no free lunch in external ballistics. What you gain somewhere you will pay for elsewhere, velocity gains = reduced barrel life..........more muzzle energy = more recoil = more ruined wood stocks.
I will leave you with this thought........there is no need in the hunting or target shooting world out there, for any more cartridges or calibers. Any purpose that a firearm could be used for, there is a suitable cartridge for it. HOWEVER if you just want to have fun and waste a lot of money and gain experience and knowledge, fly at 'er. There is a great deal of satisfaction taking game with a rifle/cartridge combo you designed and possibly no one else has ever used on game before. I hunted several species in Texas with a 7mm based on an 8X68 case improved to 30 deg shoulder. I'm fairly certain, given the difficulty of obtaining brass that no one else has done exactly this before and taken game with it, so............it was worth it to me.
Douglas