Under carriage/under chassis pics..

Resto Rick is selling a urethane top coat that is supposed to match the factory dip primer. But it's $200 per gallon, not including the reducer.
http://www.restorick.com/proddetail.asp?prod=PntPolyDipGray

Here's another option I'm considering. It's $150 per gallon including reducer. And it's available in quarts too.
http://www.tcpglobal.com/Customshop/HRF206.aspx

Looks like it might be darker though.

They also have a Light Gray
http://www.tcpglobal.com/Customshop/HRF205.aspx

Maybe one quart of each and mix them together?