[cooperhw]

Every state is different.

Check with your state.

I always go with this motto. If you don't have a title (unless your state doesn't issue them on this old a vehicle) "YOU DON'T OWN THAT TRUCK".
Someone (last registered owner) does, just not you. You may have possession, and even feel you have a "vested interest", just don't own it.

In Arizona you can also apply for title as a tow company. Just fill out a form showing who asked you to tow it off their property for free, and file for title. They will look and see if their is any body out there looking for their truck, and if not give you title.
Easy peasey, "unless, someone claims "their truck". In which case you are owed the tow bill, and up to 10 days (time allowed to file) storage.
Towing companies do it every day.