I would go with the 421 or the SD-455. The SD had the best factory rods ever, but the SD-421 rods were almost as strong. I had a '65 421 in my 'bird at one time & it was really tough. Held 50 lbs oil pressure to the day I tore it apart. It needed rebuilt badly, but would not quit! One time I drove the car to Wichita, KS and forgot to check my oil before the return trip. By the time I got to Topeka she was really making some noise! threw 2 qts. of oil in it & she quieted up.
