Is my 340 Overcarbed? Avg 5-6 MPG

You have to look from the underside to see the bushing, use a mirror. If you have instructions for the distributor, it shows bushing location. The springs installed look like a good combo, blue/lt silver.

I'd turn it up to 22-24 and reset the idle mixture and see if it's snappier. I bet it is. DO NOT drive it or hammer on it if you turn initial up without limiting the mechanical/total number. If it will handle 32 total, you could run it there as long as it starts ok when hot.

Find the point where it kicks back on the starter when hot, that's your top side initial timing limit. If that point is greater than the total number, just lock it out (see instructions) at 32* and drive it.