Was it formerly common practice to credit voice actors in opening credits?
It's uncommon, but it's not that rare.
According to the Japanese Wikipedia:

It's often to have 2 credits scene for movies, TV programs, etc.: opening &amp; ending. The general rule is, opening credits the studio, main performers, producer, supervisor, writer, etc., and ending credits all of the staffs.
In Japanese TV dramas, it's common to have the opening shown all the credit and omitted the ending in the 60's. However, in 70's and 80's, the credits started to split to opening with important credits, and ending with the rest. In 90's and later, it became common for the opening to only show the title, and the ending plays the theme song while listing all the credits.

However, most of 60's &amp; 70's anime OPs didn't even list any credit, only showing animation &amp; song, sometimes with lyric (unless I was apparently watching the non-telop version...). Even if they list the credit, it's similar to the current trend (production-related credits), but there were some which included the voice actors (out of around 25 anime OPs that I watched):

ビッグX (Big X, 1964): YouTube video


夕やけ番長 (Yuuyake Bancho, 1968): YouTube video


もーれつア太郎 (Moretsu Ataro, 1969): B/W OP (YouTube video), Color OP (YouTube video
 

男一匹 ガキ大将 (Otoko Ippiki Gaki Daishou, 1969): YouTube video



Note: 声の出演 is the same as CV (Character Voice).