Six cylinder and V8 Mustangs of that vintage were VERY different cars… Your understanding is correct. The suspension, steering, brakes, running gear are all different. 4-speeds were in great demand and were seldom found in the six cylinder cars. I think they were only available in the V8 cars…You would gain little by installing one…
What limits the performance of the Ford six is the cylinder head with it’s cast in intake manifold. That head/manifold simply does not breath very well. However, aftermarket heads were made for that engine and a few hot-rodders would cut off the “log” and with some fancy welding and machining, install 3 Weber side-draft carbs along with other head work… These engines found their way into USAC Midget race cars of that era.
It was a lot cheaper then and now just to buy a V8 powered car than to spend a lot of money on improving the six…But the six is a better balanced car and they handle better everything else being equal.
I found this in WiKi…Looks like someone is making a trick aluminum head for these engines…
“The 250 cu. in. Inline Six was an engine option offered in 1969 in the Mustang, and 1970 in medium-sized Ford cars (Maverick). The 250 was a stroked 200, made by changing the stroke from 3.126” to 3.91". Output was 155 hp (115 kW) in the Mustang, and became the base engine in 1971. Power was re-evaluated at 98 hp (73 kW) for 1972 (due to power rating changes) and 88 hp (66 kW) the next year. The last year of production for the 250 was 1980. This engine had seven main bearings, and can be identified by the five freeze (core) plugs on the side of the block. The block uses a low mount starter and six bellhousing bolts, sharing its bellhousing with the Windsor V-8s 302-351W, late (1965–68) 289, 351 Cleveland, early 4.6, and the 240-300 CID Ford Six.
Restoration and restomod status
The rising cost of fuel has increased the interest in the Falcon six design, and the engines have been gaining in popularity in recent years. Many Falcon and Mustang owners who used to replace their car’s original I-6 engine with a V8, are now keeping their inline sixes for the better fuel economy the inline six motor offers. A number of restoration and performance parts vendors now stock parts for people restoring their inline six motors.
Due to increased popularity of the Falcon I-6 engine, the desire for improved performance, and difficulty in finding good quality used parts for an engine that has been out of production for 25 years, in 2005 the company Classic Inlines designed a modern aluminum head based on the Australian 250-2v head but with raised intake ports, high swirl combustion chambers, and larger valves. One of the features of the new head are pads cast into the top of the intake runners, which allows owners the ability to machine the head to accept modern electronic fuel injectors for direct port fuel injection.
Not only does the new head improve power output, but also the high-swirl design improves fuel mixing in the cylinder for a more complete burning of the air fuel mixture, resulting in reduced emissions output.