ford has an OEM website but I don't think they don't go all the way back to 76http://www.motorcraftservice.com/vdirs/ ... d=&gutsid=
On the 20 amp fuse, if the original fuse was a 20A and it blows, they also make a 20A circuit breaker that will reset itself. But, If it does blow right away, you're going to need to find the short in the circuit. With the fuse NOT in the system, what doesn't work? What is the fuse for? Do you know what circuit the fuse is protecting?
If you need time to find the short and don't want to burn out a ton of fuses, use a test light between the two fuse terminals in the fuse box. Using the light bulb in place of the fuse will allow you to energize the circuit but because the light bulb draws current it won't blow out like a fuse. Then trace the wiring from the fuse box to whatever it is supposed to go to. Finf the short to ground.