I recently found a set of tailgate hinges for my 1965 D100 tailgate. The truck has a long bed, the pie plate front lights and the rear brake lights are rather oval. Anyway the tailgate brackets are about three inches long, have holes on each end and what looks like a cup in the middle for the tailgate to pivot on.

Here is the question. I go to attach the brackets to the end of the bed and I could not find a screw that would fit... 5/16 too small 3/8ths fine thread too big... 3/8ths course thread I could get started but only for the first turn or two. Does anyone know the correct size? We are moving and I would like to get the tailgate attached asap. Also, are there any tricks to attaching? I have never attempted before.

Thanks for your help.