I'd do the Hemi with an automatic in the 66 300 M - as a stable mate to my Monaco 440 4-speed.
So - to back up what you're saying John, and to give others a bit more background to my claims regarding this particular car and the chance it could have come with a Hemi (even though we know that NO C bodies had factory hemi engines), here's the story once again:
There was, according to the original owner of my Monaco (which was ordered with a 426 Hemi and 4 speed and the order was accepted and deposit paid in June of '65) a Chrysler trade fair was held in May of 1965, where it was announced that most definitely there was going to be a 66 300 M with the Hemi option, as the 426 Street Hemi with hydraulic valve train was scheduled for release to publicly available cars. However, the Hemi M project was scrapped after the bean counters deemed it unworthy due to challenges the test mules went through, proving it would be too difficult, costly and not a good return on the investment in tooling and engineering for an extremely niche market option. Think about that 65 Town and Country with 426 hemi, 4 speed, Disc Brakes (you can see the special master cylinder in the pics below, but it has the wrong booster on it - likely a replacement), AC and all the other options that was made as a dealership conversion somewhere in the states (I forget where) as one of the test mules for the 66 Hemi M.
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There are far too many factory parts and well placed engineering on this car to have it be some backyard conversion. However, with the crazy amount of tweaking and re-engineering of parts just to fit the Hemi and give people the options list expected in a Chrysler (like AC and power brakes), AND the known issues with Hemi durability (even with the new for 66 hydraulic valve train), plus the need to add a decently attractive warranty to an engine option clearly designed for racing, the 440 was hands down the winner in this case and so it's a no brainer that the option was cancelled - but the option for a 426 Street Hemi was very much in consideration during 1965 planning - so much so that it was announced at a Chrysler trade fair where high volume dealerships sent representatives to this company gathering to see what was coming out for the next year. The person who went to this trade fair from the Calgary dealership where my car was ordered from was good friends with the original owner of my car (who also worked at the dealership), and spilled the beans to him in May of 65. This prompted my guy to immediately special order the Monaco with the options that would have been available in the 66 Hemi M, knowing that in many situations, especially for the high volume dealerships that sold high profile cars etc., Chrysler would build cars with options that weren't necessarily on the option sheet. Being located in Calgary Alberta (where the highest number of Hemi cars were sold in North America), the dealership in question was one of those where special order cars were worthy of consideration, and the dealership processed the order with the intent of having this car made as ordered. When Chrysler scrubbed the project, the dealership didn't want to lose the sale of what was a very expensively optioned car, so they substituted the 440 in place of the hemi for my guy (and didn't tell him), who was understandably pissed about it when the car came in... then he took it for a drive...
This dealership also sold all the pursuit cars to the RCMP for those years... so the engine that was substituted was not just a run of the mill 440, but was built for police duty on the special engine assembly line - factory balance and "built to design tolerance" which was the Mopar term for blueprinting.