I HAVE A 390 BORED .080 OVER AND 428 CRANK AND PISTIONS. BUT HAVE STANDERED 390 HEADS. IM REDOING THE MOTOR AGAIN BUT UPGRADING THE HEADS THIS TIME TO ACOMADATE BIGGER VALVES AND CAM LIFT. I WAS GOING TO DO A 429 TO PUT IN MY 69 FORD PICKUP BUT I DONT HAVE ALL THE PARTS TO MAKE THE CLUTCH WORK(Z-BAR AND BLOCK AND FRAME MOUNT) SO ILL JUST REDO THE NOS 428. AS FOR THE WEIGHT OF THE 390 I BELIEVE ITS AROUND 600+ LBS FULLY DRESSED DUE TO THE EXTENDED SKIRTING ON THE BOTTOM OF THE BLOCK(OIL PAN MOUNTING) I ALSO PERSONALLY BELIEVE THAT THE 390 AND 460 GOT PRETTY CLOSE TO THE SAME FUEL MILEAGE (0 TO NONE)NEITHER WERE BUILT FOR MILEAGE.I HAVE DUAL 4'S, CAR 4-SPEED,3.55 GEARS AND A CRANE REV LIMITER ELECTRONIC CONVERSION THE BEST I HAVE EVER GOTTEN WITH THE TAIL GATE DOWN 70MPH, 3000 RPM IS 16MPG. IN TOWN I GET AS LOW AS 3MPG(DUAL 4'S AND IM NOT AFAID TO USE THEM )AND AS FOR SHUSHCANBOB NOT SURE WHAT YOUR TRING TO DO? ANYWAYS THANK YOU ALL FOR YOUR HELP.