Need some advice 318 POLY vs 318 LA

64-5 273's used 318 poly crank and rods, timing cover, brackets and pulleys. That's why the early 273 alternator bracket has two sets of head mounting holes: one set for the 273, other for the 318 poly. Also why the early 273 timing cover has one unused bolt hole on the driver's side - the poly 318 used it, the 273 doesn't.