You are limited with values available online.
Since there is not much information available you will need to consider all possible resources for your research.
Examples include an appraisal along with looking at comparable sales with completed and open auctions.
Below we get started looking at blue book prices.
Kelley Blue Book
only has blue book values back to 1992.
NADA Guides does provide values for classic cars and trucks online.
You will want to specify your model and options along with location to determine a value.
We've included instructions below using a [C1500] model.
Additionally, look at auction sites with comparable sales. An example is
1984 GMC Trucks
Select = [Classic Cars]
Manufacturer = [GMC]
Year = 
Model = [C1500]
Type = [1/2 Ton Pickup Shortbed]
Zip Code = 
Options = [Select]
[Get base values]
Low Retail = $8150
Average Retail = $16300
High Retail $24400