The F250 with the 8' bed would be a good choice for your purpose. However, it being a 1970, I would be concerned with the condition of all suspension and steering mountings and bushings - I would pay special attention to the Twin I-Beam front suspension.
Miles are very good in the sense of the engine not having had that much use. However, I'd like to see a documented history of service - for example; coolant changes every one or two years, brake fluid flush every year, transmission services, thermostat, radiator, hoses and belts, tire rotation, etc.
If all of that checked out OK, then have a mechanic go through the brakes and shocks and make any upgrades necessary to handle the extra weight.
Price is dependent on the truck's overall condition plus whatever your local market is indicating - I would look to pay no more than $8000.00 for the Ranger option in extra clean condition with all accessories working.
Please let me know if you have any other questions or concerns.