I am presently engaged in negotiating the purchase of a '66 200 with the M121 engine and 4-spd transmission. This 200 also has also the rare floor shift. I will be facing an engine rebuild and I would like to make some performance modifications that will allow me to retain the stock appearance of the engine (with the exception of the air filters}. I am considering milling of the head as well as a port and polish, a higher-lift camshaft and matching springs, rejetted carbuerators, balance all rotating assemblies, dual exhausts and lower gear ratio for differential.