My suggestions: If the engine has an aftermarket cam in it and the prev owner advanced the cam with a degree wheel then you will need to find a "sweet spot" in the static timing where the engine likes to run. My 396 has a Summit cam from the previous owner and it likes the static timing pretty advanced, even though the rest of the engine is stock. it does not like 4° BTDC.
Also, you may have an overlap issue depending on your cam grind, thereby causing fluctuating vacuum signals to your carb. That jumpy idle issue could be an incorrect power valve issue within the carb. But,
if you have large fluctuations in the vacuum reading while idling, you may even have a bent valve that isn't closing. ...Or a cracked valve or worn seat. The valve could be allowing air to leak past it. This could also cause the power valve to be operating in an erratic fashion that would affect your idle speed. It would be best to find the cam card and know the specs. If it is a high lift/duration cam then your carb may be too small and someone tried (and failed) to compensate by turning the mixture out so far. But if you hear a valve ticking and attempting to adjust it doesn't change it that would be the first
place to start.
Also, you might want to turn the mixture screws in more. 1 & 3/4 turns out sounds kind of fat on the mixture. You want to work within a range of zero to 1.5 turns out- max. Your best range should be between .75-1.25" turns out if the carb is giving the correct CFM for the cam grind and it is jetted correctly. The mixture setting will best be observed by reading the plugs after driving with different adjustments. Many novice tuners think turning the mixture way out is better but this is not the case. If it's idling in the garage and your eyes are burning it's a sure sign it's too fat.
The poor acceleration could also be from the mixture too fat, but can also be a number of other things. I'm sure that's not what you want to hear.
If you don't have a harmony of correct mixture, correct CFM, correct centrifugal timing advance and a cam that has a power band too high in the RPM curve, combined with a tall rear end gear you have all the makings for poor acceleration. Also, if that Holley has vacuum secondaries, you may have to purchase a spring kit to match the engine so that the secondaries open sooner.
It's tough trying to fix issues from previous owners. Don't forget, you bought something someone else sold and may have given up on due to inexperience. Those can be excellent buys though if you know what you're doing.