Backfire through the carb is timing, usually. Go back through, get TDC on your compression stroke and make sure that your number one plug wire is lined up to the rotor with the motor in this position. Gas in the oil is usually not a good thing, but could come from a number of places in a new motor. You may have flooded it and it ran down before the rings have set, Ive seen fuel pumps go bad and leak into the oil galley.

Change the oil, bump the engine until you have number 1 cyl in TDC on compression, pull the cap and look at the rotor, the end that fires should be directly under the terminal for number 1 spark plug. They usually run a tooth off, but two would make it spit a sputter. You may be out 180, that is have the #1 wire hooked up at TDC on the exhaust stroke.

Just sounds like timing to me.