You MIGHT be able to squeeze a 200 I6 from a Fairmont (Foxbody Mustang) into a II, but you'd be looking at fabbing up engine mounts and length would be a bit of an issue...
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...
;-)
The 200/250 isn't much bigger than a 2.3, I'm not saying its the same size just that the two extra cylinders shouldn't be that much of a problem. Some company in Australia makes a crossflow head for the 250 which is the same as a 200. which leaves you with more hp possibilities. Theres always moving the rad H1 style, above the engine.
Most all of the I6s are pretty tall too. Have enough trouble with the 250 I6s in Mavericks and their engine bay is taller than a II. It takes a Carter RBS 1bbl to get the 250 I6 to fit under the hood of a Mav and still retain an air cleaner. The 200 I6 is a bit shorter and thus you'll see them mostly running the Carter YF 1bbl (a much taller carb).
What's wrong with a V6? Always wanted to try a 4.0L into a V6 II or you could go 3.8L and that should bolt up to a small block 302 bellhousing without much trouble at all.