I see more 1965 200's than newer ones in Facebook Marketplace but I don't see any on Craigslist. That might just be where I am, on the West Coast of Canada. They probably have them in the bone yards though.
1965 used the small bell housing but if you're keeping the original transmission, a dual pattern block would work too, so you can look at later engines and if it's dual pattern, you can change the dowel pins and use the old transmission, flywheel/flex plate, clutch, starter, etc.... I don't know when they stopped producing the dual pattern block; it was introduced in 1966 and I think they made them into the 1970's.
Check the classifieds on the site too. Good luck.