Years ago I came across a single pic of a MII with a small straight six in the engine bay. There was no detail, so I've always assumed it was the 200 I6. It was really crowding the radiator support, so your radiator would probably have to be in front of the support or would you have to use an electric pusher fan for engine clearance.
It woudn't be much fun without some HiPo parts though, and I don't think there is a HiPo head for the 200 like there is for the 250 (the Austrailian OHV and OHC heads). The 200 has the integral intake manifold which severely limits performance improvements.
If you could solve the intake issue, the 200 wouldn't be a bad-sized engine. And you could make some money (with the classic Mustang folks).
Or you could use an I-5 or I-6 from Volvo and claim it as a Ford part due to the (former) ownership...
;-)