My experience has been (with multiple 64-66s slant 6's) that the mechanical automatic chokes on the early slant 6's 170/225 (both Carter and Holley) work just fine (even on cold days) (if the spring is in reasonable condition, the choke is adjusted properly, and everything else is functioning including the choke pull-off and especially the heat riser in the exhaust manifold. I replaced a worn out choke once and the new part did not operate properly at first, but worked just fine after I readjustend the screw within the choke to slightly change the (rich/lean) setting. The mechanical choke on my 65 273 with Carter carb also works fine. My experience is that the chokes that seem to be problematic are the ones which include the build in heater and operate with a timer (mid 70's? - 80's).My comment was only to the fact that Dodge auto chokes on the /6 225 was crap and I know this fact personally as I experienced it - even when I bought a brand new one and hooked it up, it never really functioned properly on really cold days.
As previously mentioned you are probably having a hard time locating one because the Holley carb on your car is probably a replacement carb. The choke for the Carter carb with 273 is available on Ebay. You might be able to use the Carter carb choke if you slightly alter the linkage. Or put in a totally manual choke or switch back to the original setup with the Carter carb.