FYI Early 273's and probably the poly 318's do have a smaller converter snout register. If '66 is the last year for the smaller register, it would probably include all poly 318's. Otherwise, "regular" 318 cranks can be rebalanced.
The forged 318 truck cranks had solid rod journals.
Cast cranks tend to be lighter than their forged counterparts which can help in some racing applications. 340's had cast cranks in later '72 and '73 engines. But adding weight to compensate for heavier 340 pistons does take away the lighter advantage and cost a lot, unless you end up using lighter aftermarket 340 roda & pistons. While still strong, cast is naturally weaker than forged as well.