This is some general info on what was included iin the Trailer Towing Heavy Duty package on trucks in 1975 - 1980
The code is YC3
It included:
55 ampere Alternator
70 ampere Battery
Increased cooling
HD Variable Load Flasher and Wiring harness
Heavy Duty shocks, front and rear
four 15 x 6.60 wheels
Hope this helps