Salvage Yard would be your best choice.
p265-75r16
Loosen the dash and pull out the complete heater box after you have disconnected the heater hose from the firewall...........
In the car
its in the glove box
Six foot. if it is a Z71. But some do have an eight foot bed.
1988 thru 1998 look basicly the same.
take out the glove box
How do you replace the heater blower motor resistor on a 1998 gmc serria 1500 p/u
On the passenger side in the black box
behind the glove box, on top of heater compartment
Look behind the glove box. under dash.
Its in the glove box. Push the power distribution button on the right side.