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.