As said above, sometimes less is more, but even if you're overcammed, ditching that distributor for one with vacuum advance, and moving to a Thermoquad carburetor would be a major step in the right direction. Tighter converter (stock 340 is around 2400 stall) and 2.7 or 2.9 gears would help a bunch too. It goes without saying you'll need to have the carburetor and advance curve fine tuned to get the best results. No reason why a well set up 340 couldn't be knocking down around 20 MPH in an A body.
Jim Linder runs a shop specializing in distributor/ignition work in Speedway. Goes by GMC Bubba on the HAMB. Might be worth looking him up.
