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Is it a stepside or fleetside? What year? You can definitely make them but i would not use pine.
We made our own, we used 1'x1'x8' pine purchased from a local lumber yard and if I remember right, it was close to $80. Fortunately we already owned a table saw and router table, had to purchase a forestner bit set and then a day of screwing around. Hind site being 20/20, should have just bought them. The shipping was the deal breaker. GM used hard yellow pine from the factory for a number of years.http://www.mar-k.com/_assets/images/instructions/bed_wood_dimensions.pdf
Didn’t put any clear coat on them because that always peels off and looks terrible