Stock 340, have a -8an fuel line hooked up to a Holley 600 double pumper. Would this big of a fuel line have any effect on carb performance? Such as cause a rich condition?
I am using a edelbrock 6psi mech fuel pump

it ain't from that. More details on what you have/whats going on (idle /part or WOT etc) what recent (related) changes. The more info the better

To small fuel line size can restrict the fuel flow, bigger size won't make the pump make more pressure so no, it won't make the carb. run worst Which Holley list number is your carb? The list number(Holley part number) is on the front of the choke tower on the drivers side Did you buy it new or used? If used post the list number so we can look up the stock jetting so you can verify if it has the stock jets or not Many things can cause a aftermarket carb to run rich on one motor and not run rich on another, similar C.I. motor of a different brand

The fuel line size being too large won't really affect the fuel pressure. Excessive fuel pressure is what can cause the fuel to overrun the needle/seats and flood the motor with excess fuel.

That being said running a -8AN fuel line is beyond overkill for a 340 but it shouldn't adversely affect it. You need to check jetting, power valves, float levels, etc. which may require a dyno or 02 sensor to get it right. Also, if you have a tee fitting and a fuel pressure gauge, make sure you aren't getting more than 6-7psi.