How can I find out my truck's towing capacity?

I have a 1979 C20, 350cc, turbo 400 hydromatic transmission, with transmission cooler and helper springs. I've been using it to tow a 2-horse bumper pull trailer, would like to get a larger gooseneck trailer and want to make sure I don't end up with a trailer that my truck can't pull.

The label inside the driver's door says:

GVWR 6400
GAWR 3500 FR
GAWR 3980 RR

Thanks!

It's hard to find a towing rating that far back. If you can find your Combined Gross Weight Rating, then subract your curb weight or actual empty weight and that's the max tow rating. Your rear axle rating will determine if the max trailer capacity has to be a gooseneck/5th wheel or bumper trailer.

I found my answers by purchasing the owners guide and towing guide for my truck off ebay. Cost me about $35 but it's nice to have the numbers straight from the manufacturer