I'm thinking the plugs were a key component. Having the carb that * I * rebuilt being another factor. Not to say the other was done improper, but I have no idea either way because I didn't do it. Also, it should be noted that I am in NO WAY or claim to be a carb expert. About that 600, though, all I can say is that this car just did not like it. None of the usual tricks worked on it.Glad that you figured it out! Interesting that it was the carb.
I doubt it was the plugs.