The 673 is a rework of the Model 7. It is limited to 2.83" overall seating length.
Yes, you'll have the seating depth, and powder capacity issues with your new 6.5 but don't let it bother you.....
I had the opportunity to get a .350mag in one before the shop went out of business. I really wanted a 6.5 but have never seen one. (would have liked the .350 but already have a custom .338/06 so considered it redundant, but then again, I have two .257Robts. and a .257wby, and two .30/06 B/A, -one left handed, so that excuse dosen't really wash....)
I have a Rem Mod7 in 7mm-08 that I originally intended to rebarrel to 6.5-284 or just rechamber .284win. Made mistake of shooting and chronographing it first. It's now my favorite rifle/cart. (2,900fps w/140gr bullets)
Bottom line: With newer powders such as H4350, RL22, IMR4007, and RL17, and IMR7828 (especially with the "short-cut versions) you have some wonderful options for maximizing the 6.5rem-mag.
John Barsness ran some excellent articles debunking the myth of "limited" powder capacity on the very similar .257Roberts and .300WinMag regarding bullet seating depth vs. powder capacity limitation, and short necks.
I've got both a Custom M98 Mauser in .257Robt. with the so-called 3" chamber and throat. And, a Ruger M77MkII with the SAAMI "Remington" short throat. Although the Ruger barrel isn't quite as accurate as the E.R.Shaw barrel on the Mauser, it gets equal or higher velocities with slightly lower powder charges and seated to SAAMI lengths (I.E. 2.775-2.825" lengths, the latter the seating depth for Berger 115gr VLD slightly off the lands where it gives "it's" best accuracy- under MOA for an otherwise 1.5-2MOA rifle).
Example- max load of 45.0gr of IMR4831 w/115-120gr bullets.
Mauser- @ 3.015" oal; gives ~2,800fps
Ruger- @ 2.800-2.825" (bullet dependant); gives 2,975-3,050fps! or .25/06 velocities. (of course at .25/06 pressures too)
I see similar trends with IMR3031, both H and Imr4350, H4831, and RL22.
Someday I may run across a Mod673 in 6.5 and I'll probably buy it on the spot.
Good luck and report back!