Triangulated 4 bar rear end spring rate

I am installing a triangulated 4 bar rear end in my '48 KB2. Based on '64 chevy 10 bolt rear end and basically stock type bars (custom tubular, adjustable). I will be using QA1 coil overs. Anybody have any suggestions on recommended spring rate? I am thinking 275# - 300# spring rate.

Light duty, 1/2 ton pickup type hauling at most.

Thanks in advance for any input.