[RichardJ]

If you have a '71 C20 that weighs 4000 lb, it has been on a serious diet or you should find new scales.
The image below is from the 1971 GM Heritage page.
A C20, V8, fleetside curb weight is 4162, which includes oil, water and gas.
That is with a 307, 3 spd on the column, no rear bumper of any style, five wheels, but only four tires, one muffler, one tail pipe, and it does include two door mirrors. My '67 was only allowed one mirror!
Other than front bumper, two fenders, hood and tail gate, anything else on the truck is going to kick it up to a more realistic 4600 lbs.