The cover on the 266 extends back over the the top handle, which unlike the 670, is bent inward (the handle) so the cover clears. The 670 cover ends along the front edge of the handle and could be used as is on a 266. If you cut the 266 cover extension off it would work fine but wouldn't look quite the same as the 670. Or alternatively you'd have to also put a 266 top handle on the 670 (and then the 266 brake bar would line up at the same angle too, without bending it).
My 670 needs some brake hardware and I figure by the time I buy that I'd be close to the price of replacing my functional but cracked one with a $50 scratched up but complete feebay one. Other saws keep sucking down the parts funds though...
Bill