just got a 66 gmc project got some questions

[QUINN66GMC]

Hi my name is Quinn and i just got my grandpas 66 gmc it is currently a flat bed with a 1 ton rear end in it i want to change it back to a 1/2 ton step side. it has been sitting for a couple years. i was wondering what i should do to it before i start it i was going to start by draining the gas tank and changing the fuel filter but other ideas and things to check would be much appreciated. also i want to sell the flat bed and rear end also the cage that protects the back cab any ideas on a price range for these things. i am 24 and don't know a lot about these older trucks so any tips on this build would be greatly appreciated. sry the pics are upside down i cant get them to post right side up and it needs washed really bad.