Conductor - 2 or 4. 4 comes in effect as 2. You want to order 4 and wire it in series, which it will arrive ready to do. 4 allows other switching options later and keeps maximum resale value. 2 is series (normal humbucking) only. The pickups you have in there are 4cond, so it should be easy.
Leg length - the length of the brackets that the pickup is screwed in with. You want short.
Spacing - the distance from low 1st to 6th pole. You want standard/non-trem.
P.S. 456s are great bridges, good choice.