Welcome to the forum! There is no "blue book" for 50 F6 trucks. Value is highly dependent on location and condition. For most guys, F6's are better used as parts trucks, and even those who have a desire to restore the big trucks, they don't tend to spend a lot of money on them. If you have a rusted hulk that doesn't run and has no title, you're looking at scrap value; maybe a couple hundred at best. If it's nice, rust free, running and driving, you might get a couple grand from the right guy. Depending on condition, yours could be anywhere in between. Pictures could help, but for F6 trucks, for which there's a ton of out there just like yours, that's pretty much the reality.
My truck was a fire truck 7500 miles been indoors all of it's life. Has lights and siren. No rust and all original. Has a flat bed on back. Runs and drives great. Will post some pictures when I can.