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 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
1978 Mustang II with 4 speed... what kind of fluid goes in? 
