All automotive are LH, left hand.
Many marine are RH, right hand, even in single engine boats. The RH prop torque thrust in a single engine small boat helps trim it back to level when one person is aboard (right side heavy).

Are you building a new motor off an automotive core? If so,
-reverse the pistons on the rods, not the rods on the crank
-polish out LH rear main knurling and use a neoprene seal not rope, if you have RH knurling leave it and use a rope seal
-use a reverse cam core, SB may be available new, BB isn't, besides location of lobes, the gear is cut backwards
-use a reverse dist/oil pump drive gear, this allows std oil pump
-use a marine thrust block between block and dist, the LH drive gear walks down against the block bushing, the RH drive gear tries to walk up so a thrust block is needed
-marine dist is special with spark arresting features, it does rotate the same direction as LH, just the wire order is reversed from 1-8-4-3-6-5-7-2 to 1-2-7-5-6-3-4-8
-marine carburater, alternator and starter are special with spark arresting features, carb uses spark arrester instead of an air cleaner
-water pump depends if it is raw water or heat exchanger cooled
-and of course the exhaust manifolds are water cooled

What model Century? What trans, Borg Warner or Paragon? Any pics?