1967 273 4bbl with Edelbrock 500 CFM Carb

Why is the idle set to 800?

Lets talk about idle;
The faster it idles and/or the more idle-timing it has, the more power the engine will have at idle, and so
the harder it will bang when going from N/P to in gear.

The further the throttle is opened at idle, The more fuel will be discharged from the transfers. This will make the idle rich.
In compensation, you might think to close the mixture screws. That's fine, now it will idle. But as soon as you tip the throttles in, the entire slow-speed system is lean, because that system depends on the mixture screws being about in the center of their adjustment range.
The opposite is also true, namely; The more closed that the butterflys are at idle, the less fuel will be discharged from the transfers, and so in compensation, the mixture screws will have to be opened further. But now, as soon as you tip the throttles in, the entire slow-speed system will be rich due to the too-far open mixture screws.
There is a very narrow idle-adjustment window, in the which the transfers and the mixture screws are fairly optimum. If, at this setting the idle-speed is too slow or too fast, you adjust the speed with Ignition advance.
The more idle-advance that you give her, the faster she will idle. And
the less advance that you give her, the slower she will idle.
When all is said and done, you have to check your power-timing and make sure it is not so much as to cause detonation. You may have to modify the distributor to get that right.
Happy HotRodding.