I want to buy a 1977 4x4 . The guy said he have a short fleetside box and a stepside box.what is a fleetside box? Thanks.

 

1977 4x4 short box Fleetside

 

"Fleetside" is a GM term, Ford's equivalent is "Styleside" just like the "Flareside" is Ford's equivalent to GM's "Stepside"

Flareside/Stepside=bed wheelwells outside of bed.
Styleside/Fleetside= bed wheelwells inside bed.

Like redroad posted, that's a picture of a Stylside aka Fleetside bed. (That's one nice looking truck BTW).