Yeah, one on center and second with the amount of off-set, same on the opposite end at exactly the same vertical axis.
What you are making will "work" with a relatively short crank, where you don't need to support the other end with the live center in the tail-stock.
Also the overhang that your fixture will case it may not feasible to free-air machining because of ( chatter ) and end support would be necessary.
Now, being lazy by nature ... I have not read all the postings and since you've been wanting to make steam engines, I assume that is the why of your creation.
So if I am shooting blanks please forgive me.
Here is a video that shown what I know about machining a crank.
http://www.youtube.com/watch?v=3YfTtGCsiD8
Jose
There are no problems, only solutions.
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Retired journeyman machinist and 3D CAD mechanical designer - hobbyist - grandpa