Thought I posted this here, but I guess I forgot:
I've found a good source (via Wikipedia) for the Electra, at least for early models:
http://www.flightglo...955 - 1595.html
To boil it down simply,
In a one class 3-2 one class config, 85 is good for a realistic 50s-60s time period, and for a 2-2 one class config, 66 seems to be normal. What I've used in the past, however, is a two-class config, where I removed the 3 row front cabin, and replaced it with a 2-2 2 row C cabin.
In short, for early Electras:
66Y
85Y
8C/70Y
I'm sure there are airlines that used more seats in later years, Reeve Aleutian comes to mind, but I can't find anything definitive for them.