Tim, the last time I bought a good 312 crank (I think 3 years or so ago), I paid $163 and some change, and that was on ebay after all the bidding to boot!!! "Cactus" has the crank now in his built engine that had that coolant leak issue...I tried to get another block for him, but don't know how that turned out.
As of right now, I want a 312 crank too...
but the mains aren't a big deal because I am going to turn them down to 292 size.
$150 is more than fair. Cranks are always a "pig and a poke" anyway......Who will ship it across the country with a fail safe guarantee on an original part that old anyway? Hope you find one, and $150 is fair price. I had one on the line for about $175 the other day, and ones within your specs, but I could never get the owner to respond to my contacts, and now its been sold.
Daniel JessupLancaster, California
aka "The Hot Rod Reverend" 
check out the 1955 Ford Fairlane build at www.hotrodreverend.com