Hey partime....
I'd spend it on making the truck as reliable and dependable as possible. Add all new brake parts, replace questionable seals, put on new tires & add a spare if needed, belts, hoses, clean out radiator, new relays, new glow plugs, etc. Whatever it needs......
If it's your daily driver, I would focus on the things that would give you trouble or cause you to be stranded. You can't make a truck 25 years old 100% reliable, but you can eliminate as many things as possible that could cause the truck to quit and make you walk home.
Keep some funds in reserve in case something comes up and you need to fix it fast.