I remembered turbothis' .303 conversion on theakforum:
The AK Forum :: Log in
He spliced a longer extension into a pair of milled stubs, extended the bolt carrier and bolt, and a few other mods.
rahat did an 8mm Mauser using a Coldsteel flat and modified AK bolt and carrier:
Romy RPK to 7.92 x 57mm - Militaryfirearm
Another 8mm thread here:
Coldsteelsolutionsincorporated.com has PSL flats with the side stiffening plates for $50.