I just replaced my 600 cfm edelbrock on my basically stock 307 with a 500 cfm edelbrock and its surging at idle and really bad when put in drive. What are the possibilities, is she too lean or are my floats messed up..

I know that im posting this again but i need some help cause i need to get my car back up and running very soon. Thanks for any help

Check for vac. leaks.
If it smells rich then your floats may be to high.
Disconnect the vac. adv line temporarily to lessen the surge if it's hooked to manifold vac.
You could swap carbs again to see if it is the carb.

why the carb change?

nothing wrong with a 600 on a 307ci.........

idle mixture screws turned in too far?

Possibility that the jets are too small.

Edelbrock is really good at messing up the float level at the factory! Ya might check that, it doesnt cost anything, put the float level at 7/16" and the float drop at 1" - 1 1/4"

well i want to get some better gas mileage and if you do the CFM formula with a dual plane intake( x 150%) than it comes out to 485 so technically a 500 will run the best. And i can already tell a differnece cuz she revs faster and smoother. She is just surging and im pretty sure that its the floats.