a rusty, non-running truck that has been sitting in a field for 30 years is not worth a whole lot.
it will take a lot of work to make it into a presentable truckrod.
that said and since it's family, offer $300 to $500 for it and see where it gets you.
you can buy a running pos truck for $2k to $3k
check out the 67-72chevytrucks.com
forum, they have a very active 47-59 chevy truck sub forum