Re: Carb overflow
I had a similar situation with a Holley but it was cause by a fragment of PTFE stopping the needle seating properly, and because I had parked on a downhill slope the contents of the tank drained into the (front mounted) motor. God knows how it got past the electric fuel pump but it did. I learned from this, that when you put PTFE on a fuel line connection avoid putting the tape on the first 2 or 3 threads!
When I tried to start it all I got was a heavy thud and I'm lucky it didn't try to fire else I would have hydraulicked the motor.
All I did was to drain the sump, change the filter and refill with fresh oil - no problem. I think all fuel is contaminated with gasoline to a certain degree - and I think your idea of flushing the system is a good one plus a shot of Motor Honey or STP down each cylinder could not do any harm either and spin it over without the plugs in...