There are many coil spring calculators on the internet - from very simple to 'why do I need that information'. For the most part, you need the wire diameter, free coils and height and the angle they are installed. As a general rule, the Speedway/AFCO suggested chart works just fine for street rods. I have a TCI chassis under my '31 - they supplied springs that could have held up my 3500 kg pickup truck and it rode like one!!
My car now has 325 pound capacity in the front and 200 in the rear and is probably similar in weight as yours.
Current adjustment, about one inch +/- of thread:
The AFCO Chart