There was an optional 145 amp alternator offered in 1978. Roger Black had one on his coach. I researched it one time about 15 years ago after seeing it on Roger's engine when we swapped it out of a GMC transmode people mover.

Try finding a 1978 Z87 GMC Motorhome parts book. I have it in a .pdf file and it is 32.2 meg. Billy might all so have it on his bdub web site.

It is in section 7.02H with a part number of

7.02H GENERATOR—145 AMP DELCOTRON (1978)
1117148 GENERATOR-145 AMP (w/RPO L04)

It might be a one wire alternator. Check carefully before using.