holleys typically come jetted rich. Check the size of the front jets and get some jets 2 and 3 sizes smaller for testing.
I would also recommend that you buy an adjustable vacuum advance canister. summit racing sales them for 25.00.
Set the the idle timing to 20 degrees, this should give you about 38 to 40 total revving at 4000 rpm. Verify this!
Then use the adjustable vacuum advance on the manifold source with 10 more degrees of timing. When you plug it in, it should be at 30 degree at idle and around 50 when you rev it in park.
Then, adjust the idle speed to 800 rpms, and then adjust the idle air mixture screws for highest rpm.
You should be getting 10-11 mpg in town and 15-17 on the highway (depending on speed).