I'm pretty sure the 4-spd. doesn't use ATF of any type (Ford manual trannys didn't start using ATF till the mid-80s). It likely would use 80w90 gear oil or gear oil of a higher weight. I couldn't tell you what weight for sure. (The C4 automatics used in IIs don't use the typical "Dexron/Mercon" ATFs either, they require Type-F).
You may be able to have someone look it up for you at a parts store, most counter-jockeys won't know how/where to look (I can't believe how lost parts store employees look without their hands on a keyboard) but if you find someone that knows what they're doing they'll be able to look it up in the lubricants guide that Castrol, Valvoline, or Mobil provide them (most parts stores have one from one of those brands, usually stashed somewhere in their paper catalogs behind the counter collecting dust).
__________________ 1976 Ford Mustang II Ghia: 302 (World Products Heads, E303 cam, Edelbrock intake and carb, Dynomax headers, Art Carr flexplate, Pro-Comp distributor, Flowmaster Super 44s, Summit Radiator.)
Also the PROUD owner of a 2003 Dodge Ram and a 1975 Chevy Scottsdale Pickup. American, Rear-wheel-drive, with a V8, that's how I roll. | 
1978 Mustang II with 4 speed... what kind of fluid goes in? 
