Interesting.
Are you sure the Holley is the original Dart Light unit? Sounds like your timing chain has had it. 20 mpg with that car is pretty low.
Check the
list number to see if the carburetor is the correct one. If there is a Holley 1940 or Carter BBS on there, it's not right.
SlantSixDan up on slantsix.org had some special Holley 1920's at one time, NOS. I bought one for the build on my wife's '63 Dart GT. I have the correct Aluminum 1 bbl intake. One that isn't cracked in the plenum floor. (A rarity.) I've got a Feather Duster 2-1/4" head pipe that came off when I installed Dual Dutra Duals into twin 2-1/4" head pipes on my drag car.
Also check your distributor and vacuum advance can to make sure they are the correct ones. The vacuum amplifier that runs the EGR may have problems. If it isn't there, you sure don't want to run the Feather/Lite distributor curve. You'll jack the Cylinder head temperatures through the roof with 52 degrees total timing and no EGR.
Got some pics of the engine compartment? Unmolested Feather Dusters and Dart lite's are cool! And Rare!
CJ