I would recommend something in the 300-400 range with 450 as an absolute max. Stock coil springs are between 269 and 300. In fact even stock springs will still provide excellent handling if you go with performance shocks.

 

I just went through this for my 70 FB. I ended up on Mach I springs that are 428 lb/in if I remember right. The stock springs for my car were 289 lb/in. I think it was Daze that pointed out that National Parts Depot's online catalog lists the spring rate. Once you find the stock spring for your year and model car, add it to your shopping cart and there you can see the spring rate. Performance models from you car's year probably had a higher rate.