If you’re doing rifle-length gas 20”, then the port will be between what a .260 Rem and .308 is.
I’ve seen .094” ports on 20” .308 barrels from Krieger.
Some shops have used .086” ports on .260 Remington (less bore volume, slower powders, higher port pressure).
There isn’t an established standard for either, but a few shops who know what they are doing have done the legwork years ago on different chamberings in the large frame guns.
Accuracy Systems out of Colorado has done a lot of 7mm-08s.
It will also depend on what projectile weights you plan to shoot. I think 7mm-08 is a good idea if you can find ammo for it. Earlier during the first months of the panic, you would still see 7mm-08 on the shelf but .308 gone, so there’s one reason. Prices will average higher since there is no Mil-Surp or steel case like with .308 Win.
Most factory 7mm-08 is in the lighter 120-150gr projectile weight range, not the heavier 162-175gr bullets I would be interested in.
You can use an A5 or ArmaLite mid-length RET with a heavier AR-15 length carbine buffer in the 4.5oz region or more with a longer spring if you want a telescoping stock. You can also use a standard AR-15 carbine RET with the tiny .308 carbine buffer and .308 carbine spring.
If you use a rifle-length RET, use a DPMS or ArmaLite length rifle buffer (slightly shorter than an AR-15 rifle buffer) with 7.62 NATO spring. They have heavier spring weights for the large frame guns.
If you want it done right the first time, I would first call GA Precision or Accuracy Systems.
GAP will do you a GAP-10 G2 in 7mm-08 with a Bartlein barrel and it will run, as well as shoot bug holes.
7mm-08 feeds perfectly from existing AR-10/SR-25 mags.