My son wants a 6.8 Western but he's a lefty. Well actually left eye dominant and right handed. He shoots as a lefty. There are very few choices.

He's considering a custom build but I don't know enough to really give sound advice. I suggested a Defiance action, but they're almost as much as a rifle. With a carbon six barrel and a light stock, bottom metal and trigger it gets spend quick.

Is there enough room in a WSM based rifle to rebarrel to 6.8 without having to seat the bullets to deep to be worth the effort?

You would want to check the mag box length. If I recall, the MRC had a 3.1 inch length in there short actions which is more of a medium length action or look for a Tikka. I’m not sure of the length of other actions.

As Yooper pointed out, short action MRC 1999's can handle up to 3.125" COAL. IIRC, LH Win M70 WSM actions check in right around 3.000". Both look like they should be able handle the 6.8 Western at book COAL's.

Having said that, if he is already facing having an action rebarreled, why not just re-barrel a standard long action Left Hand rifle to .270 Win with a 1:8" twist barrel to shoot the new, long, high BC, 6.8mm bullets? He can reload to his heart's content with readily available brass. Or he can be lazy and buy off the shelf ammo when it is available. Based on Hodgdon website, it looks like he would only be giving up about 100fps in MV. Other than the tighter twist rate in new factory rifles, the 6.8 Western looks like a solution in search of a problem, to me...

JMO... YMMV...

I hear you.
I think the 6.8 is what the 270 WSM should have been. With what is available I'd probably choose differently myself. I even was thinking for the money he could rebarrel a 270 Wby and be in a good performance window, and still have components available.