You can save yourself a lot of heartache by just going with the 7STW. Readily available brass, data, dies etc.
You willl also save some barrel life which would not be too long in a 6.5STW.
On the other hand, the 6.5STW would be an interesting wildcat that few other shooters have. It should be quite capable of dropping anything from varmints to whitetails at about as far as you could hit them.
I'd use it for long range hunting and maybe punch a little paper with it, but mainly hunting with a 140 gr. Should be a very flat cart. And not very many people have one, just want something different also.
Check out Kailua Custom Guns. He made a 264-8mm Rem a few years ago, had it written up in a rag. Google the name; I believe I have it right.
Seems he was getting 3350 fps with 140's from a 26" bbl. I would definitely get a longer bbl, and just match that at lower pressures. You might get you 1k rounds out of it if you're gentle.
I built a 6.5 STW for a customer a few years back with a fluted 30" PAC-NOR barrel. He shoots the hornady 140 AMAX @ 3500 fps. If you are interested I could get you an email address and you can get info on the project from the horses mouth. I do know he has anchored a couple of deer with it.
The guy's name is Jim Kramer. He is a total gun guy. His email is [email protected]
Just tell him where you got his email from and I am sure he will answer any questions you might have. Tell him I said hi.