I agree with Big Chevy 3600
However, I'd bet that the 3100 has greater value than a 3600, even though the 3600 is rarer - the 3100 is more "drivable" for a larger number of people/buyers.
If the buyer does not want it, or wants it at a lower price, tell him/her that you checked on a Chevy truck discussion board that has more than 31,000 members and a majority of the respondents said the 3600 was more desirable. Say that the site has a trucks-for-sale section and you will take back the truck and try to sell it here.
I hope this works out for you and your family.
