340 Gets Terrible Gas Mileage
He could very well have that much timing. A SBM will start with 36 in a locked out distributor. Did it for years.
The OP needs to VERIFY his timing.
Then he needs to post some pictures of his plugs.
Bore/stroke ratio has almost zero affect on fuel mileage.
That thing should easily pull 10 MPG all around driving IF the carb and timing is right.