Found this on another site. I mentions a heavier balancer, but it was inside the timing cover. My brother-in-law had a 1954 Merc with a 289hp and 4-speed and I remember it looking odd with just a cast pulley out front.
"In 1963 Ford released the 289 High Performance which produced 271 BHP @ 6000 RPM. Somewhat stronger connecting rods with 3/8" bolts, thicker main bearing caps, solid lifter cam, screw-in rocker arm studs, machined valve spring seats, forged steel exhaust valves, a dual point mechanical advance distributor, and better flowing exhaust manifolds round out the package. Those fitted to Shebly GT 350 cars rated at 306 HP also featured a high rise aluminum intake manifold and Tri-Y headers.
The 289 HP features a high nodularity cast iron crankshaft, Brinell hardness tested to ensure quality. To prevent 4th-order harmonic vibrations from destroying the crankshaft at higher RPM, the engine uses a different vibration damper and an add-on counterweight. A portion of the 28.2 oz.in. imbalance found in the normal small block damper is moved to the additional counterweight. Moving the mass in towards the front main bearing reduces bending loads on the crankshaft. The special damper also has a larger more massive inertial ring. The add-on counterweight is 0.150" thick. As a result a special crank sprocket is used, C3OZ-6306-A, itself being 0.150" thinner than the normal 289 sprocket. The counterweight is both keyed to the crankshaft and indexed to the sprocket with a 1/8" roll pin. The images below show the counterweight and sprocket with its roll pin hole, as well as a comparison of the the normal and thin sprocket. "