Timing is always the first thing to set correctly because it is your engine’s foundation. If your foundation isn’t right, nothing else will be. GM engineers (and lots of racers) have found most SBCs run best at 36 degrees before top dead center total timing. Your distributor and vacuum advance also play a big role in this and must be setup correctly as well. What did you do for your “tune-up” on the HEI and do you know how many degrees of mechanical advance and vacuum advance it provides? What is your idle vacuum level?
Idle spec for auto trans is 600 rpm in gear with foot on brake. Works out to 750 rpm in park or neutral. Also, for the choke (not that you’ll really need it in Whittier lol), fast idle speed spec is 1500 rpm.
The best way to adjust valves is the EOIC method:
https://www.youtube.com/watch?v=Prj0GUZ7nes
If it shakes a lot, could be a vacuum leak you need to find. Or maybe your idle mixture screws need to be adjusted with a vacuum gauge for best vacuum & rpm. You need to get a calibration kit for that carb so you can dial it in well.
Just some ideas....