is the GVW the best way to tell a big block?

[71 super]

Check some posts by dantip, the gvw is the about the only way you can tell without the spid in a 1/2 ton 2wd. As I am sure you know the vin indicates only that it is a 8 or 6 after that without spid and the tell-tale parts- i.e. heat shield, dual hangers (three locations on both sides) correct parts such as remote reservoir, clamp to hold pressure hose at motor mount, correct mounts, auto cross member with guides for brake intermediate cable at trans mount, two piece drive shaft, 3.07 gear if auto. Thee are just off top of my head there are others. Might contact Paul Duncan as well he can help.