Need stiffer suspension

[doodah man]

Hi all,

For a while we're going to be using our truck like a truck. 3 days a week hauling a 3500 lb trailer around philadelphia.

I'm wondering what the options are for beefing up the rear suspension. Right now the trailer is pulling it down about 4 inches. I've read some about factory overload springs, but don't know much about em. Are there other aftermarket options? Is this even what i'm looking for?

It's a 72 C10 LWB with coils all around.

thanks in advance.