Not to sound condescending, but there is much misinformation here. I was there and have wrenched on these cars a lot, I have also read probably everything written about them including their development.
Lee Iacocca who was the original mastermind behind the 64 Mustang knew the behemoth 73 was dead. He went to his design studio and asked that they develop 2 concepts for a Mustang replacement. A return to the original 1964 plan was the thought process. One of the design studies was based on the Maverick platform, and the other "based" upon a highly modified Pinto platform. Based upon size the Pinto based study won.
The Mustang II is not on a Pinto frame or chassis. The chassis is highly modified and larger than the Pinto. Fact is, the Mustang II shares only some bin parts with the Pinto and has much more Granada in it. The 1974 Mustang II was the most engineered car in Ford's history and is on the top 10 list of best selling Mustangs to this date.
The 1974 Mustang fastback takes its styling cue's more from the 73 Torino Sport Fastback than any Pinto. None of the body panels or glass from the Mustang II will fit on a Pinto.
Engines. The 302 released in the 1975 Mustang II had the same power as the 1965 Mustang 289 the only difference being that in 1972 most US cars manufacturers changed from "gross" horsepower/torque ratings to "net" ratings. It is fairly easy to reverse engineer a net horsepower rating of 139 to gross in order to find out it is actually a 200hp engine.
Road tests of the day show that the 1975 Mustang II with a 302 compared very favorably to its counterpart the 1865 Mustang with the 289 both in acceleration and road handling. As well, the 139hp rating does not align with the 250ft/lbs number.
139hp net = about 200 gross
250ft/lbs net = about 300 gross
All of this in a car that weighed under 3000lbs.
Tires: The standard tire for this Mustang was a bias ply BR78-13 or BR70-13 radials. It does not take rocket scientist to understand what 300ft/lbs of torque would do to these cartoonish tires when pressed, and to say the Mustang II was light in its rear would be an understatement.
Gearaing - All II's came with a modified C4 (size) and from what I have read could not house a rear end gear over 3.00:1
So what we had, was a perfect light weight car for which to turn into a hot rod, as we all did before the advent of the "factory" muscle car.
When looking at the de-tuning done by Ford, the undersized 2 barrel carb, the 13" tires, the limited gearing, it is a wonder they turned a 0-60 of 9 seconds. However, in the hands of those who knew how to hot rod, these things could be brought to life very easily with just a 4 barrel. They handled much better than the 71 thru 73 boats and were really the rage of the day.
I still like them but agree their lines have not aged as well as some of the others in their era like the Camaro and Firebird.
I hope I have cleared up some of the misconception about this little jewel that Mustang purists love to hate on.