I agree with Panhead. In UT, that truck would post at $8k easy. That's double the original offer. I'd never go off of NADA. It varies classic car values by $1000's too easily. And depending on where and where it goes up for sale, it may takes months to even get an offer. The only numbers that matter are the numbers on the check with the offer.
As for restoration versus resto-mod... I venture for resto-mod. I honestly want to say one day (if I survive that long) "I drive a 100 year old truck." I also know that at some point, some parts are never going to be replaced with original. And there are those that say "Don't pour new wine into old bottles." So it's tuff to say one or the other. It all comes down to what YOU are going to do with it.