750 Edelbrock issue

1970 340 Timing.
1970 Factory Shop manuals indicate a pretty high idle rpm. 900 rpm is what I'm reading.
That 5* was to help reduce certain emissions at idle and run with a fairly lean mix for the same reason. The high idle (900 rpm) sort of made up for the loss of power.

If you have more info about the distributor on the current engine, great.
Regardless, it will be helpful to measure timing from the slowest speed it will idle, and then ever 200 or 250 rpm up.
Not sure what you have in terms of timing light, tach and timing marks/tape. If its limited then just get measurements to the extent you can.
-----
edit. Thought this rang a bell.
Discussed this last August. Might have got missed in the smoke.
1968 340 timing

Its an unknown distributor. Most important thing related to the idle is knowing the rpm the timing begins to advance. If you measure it, and find its 12* at 850 rpm, then when the idle seems to drop off, measure it again and see if its still at 12* and what the new slower rpm is.

Let me know if you need a hand. If its not too far, maybe I can drive over one evening.