My advise would be to get a 'quick change' secondary cap for your carb. This is a simple swap and will allow you to install any spring you want in seconds. This will allow to to easily fine tune your carb for any application. There is no 'right' spring for every 396 engine. Gear ratios, vehicle weight, trans gearing, converter stall, etc., will all play a role in how light or how heavy of a secondary spring you can run. With a 'quick change' cover you can simply keep changing springs till you find your best set-up. I have provided a link to Holleys website as well as a photo attachment of a pic of mine.
Quick Change kit