Do you have the pulley off the old truck engine? I'm pretty sure the trucks used a 1/2" belt. Here is a bone stock 57 truck 235 with a Sanden compressor. A 1/2" pulley has been put on the alternator but all the others are factory. All Ford/GM alternator and generator pulleys fit the same size shaft.

[img]https://s22.postimg.cc/6dz6s4l99/IMG_4166.jpg[/img]


Evan