Rough idle while in gear???? Almost stalls

[nacho55]

This 72 chevy truck I'm working on has a rough idle while it's In gear and almost stalls,But idles great while in park,and runs good while it's driving, I don't have a tac, but maybe I can hook one up for a min what should i set the rpms at?
here's the particulars on it:
Older hei distributor(could this cause erratic idle under load?)
New 350 crate motor
Rebuilt holley 600 carb w/electric choke ( seems to be functioning and open all the way)
Elderbrock performer manifold
Entire New fuel system
tank, pickup, fuel pump, new filter, rubber lines replaced and all hard lines cleaned out
Thanks for any ideas out there.
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