When to panic about white smoke

[purzell]

So I haven't had much success getting the truck to run, what with waiting for carb adapters, fixing vacuum leak, not having timing quite right while dealing with that stuff. Today I finally got the carb back on and while cranking, got it to run, lots of white smoke, but I didn't notice a sweet smell as some describe online. Of course, then the starter quit and I couldn't restart it anyhow.

This truck sat since 2008 before I got it. I just want to make sure that after changing out the fluids and replacing the starter and coil (next in line on Friday) I'm not going to blow up the motor, based on the white smoke being really bad news.

Is it unusual that a motor that's sat for five years will blow out some smoke before getting tuned up? Or is it likely to be one of the horrible scenarios: cracked something, valves, coolant in oil, etc...

Is it time to learn to rebuild a motor? It's the 351v6 in my '66 GMC.