What MPH are you running vs with the other engine? Different engine combo may require different gears and/or converter. Any major exhaust differences between the dyno and on the car? Restrictions? With the carb, accelerator pump squirter size & duration mostly influences 60' while jetting mostly affects MPH. Any carb differences between engines? New engine may be more sensitive and/or needs adjustments to vacuum secondaries if they are opening too slow or too quick. FWIW, in my experience, a properly tuned mechanical secondary carb will outperform a same-sized vacuum secondary carb by about a tenth. But a vacuum tends to be more "drivable" on the street.