Yes, get your ducks in a row with any listings you can find for sale of CL and locally as well as ebay sold listings. For IDI trucks similar to yours. I would include all 83-97 of same cab and wheelbase both F-250 and F-350. Exclude outliers like a lower prices slushbox truck if yours is manual, or a top dollar D60 truck since yours have a D50, or just heaps of crap worth barely more than scrap. Adjust the mean and median price as needed for any significant work you've done in the past year or two. You should have receipts for everything you ever did to the truck and point out in all likelihood whatever similar truck you bought would need all of that done (brakes, brake lines, all suspension and body bushings, oil leaks, turbo, etc...).
Don't accept the BS they'll try to push when they tell you you're very nicely done classic diesel pickup is worth what a rusted out, beat to hell, falling apart POS like the scrappers tool around in. They'll call their delusionally low made up number "fair market value" but you'll quickly understand it in no way represents the actual sale prices of similar vehicles and is nowhere near the cost to make you financially whole with a more or less identical truck in the same or better condition. Stand your ground on whatever dollar amount will make you whole, for them an under $10k truck (maybe yours is around $4k, don't know, but they'll offer you less) is a relatively minor amount to spend to make you whole and go away.