Heavy duty suspension?

[Bruce Alexander]

I am working on a 64 Spyder, "barn find" with 30,000 Mi. and factory wire wheels. I have replaced the 30 year old bias ply tires with new radials. With the old tires it rode like they were square. It now rides good but quite stiff. Is there an easy way to tell if it has Heavy duty suspension? like , in the VIN number? Was that a factory option?


 
...,Bruce Alexander