* Wheelbase: Longer wheelbases generally allowed for higher GVWRs.
* Engine: Different engines had different capacities.
* Axle ratings: Heavier-duty axles supported greater weight.
* Box truck body: The weight of the box itself added to the overall GVWR.
To find the GVWR for a *specific* 1987 Chevy P30 box truck, you'd need to look at:
* The vehicle's title or registration: This often lists the GVWR.
* The vehicle's sticker or placard: This might be located inside the driver's side doorjamb or on the driver's side door.
* The owner's manual: If you have the original manual, it should specify the GVWR.
Without this information, it's impossible to give a precise gross weight.