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Welcome.
All T3s are the same long action. The difference is that short action chamberings use a mag with smaller internal dimensions and a longer bolt stop to limit the length of the throw. Longer chamberings obviously have more mag space and a shorter bolt stop.
I don't know anything about chassis stock replacements, but I did a .300 WM build using a T3 in a McMillan A3 with a Bartlein barrel and I like it very much.
If you are sold on a 6.5 Creedmoor, go for it, but I would suggest that you look at a .260 Remington as that is a factory chambering and the Creedmoor doesn't do anything new that a .260 can't do. Creedmoor is just the latest trendy chambering. That would save you the cost of a new barrel.
Good luck whatever you do.
