[rockfla]

Art
Unfortunately, in the "modern fastener" world, you have a need for what is approaching "Hen's teeth" status. In the current "Industrial" world I work in, 7/16th's, by and large, is what we call a "bastard" diameter. WHY automotive industry landed on using 7/16 & 9/16 diameter fasteners is beyond me (9/16 being even WORSE). Second- the length (beyond 6") moves your need into another stratosphere. The only requirement left to send it into "deep orbit" will be to ask for "Fine Thread". FOURTH- the amount you need..... IF a distributor like me could even find them, my minimum's BOTH overall amount and quantity amounts have almost made it impossible to help people like you on "special orders/needs" and I have almost gotten to a point where IF I don't stock it then it's a "NO" on ordering it UNLESS it is in carton quantities and a company I order from on a daily basis to meet order minimums (most of mine now are $100 to $500).
AS Kube astutely stated above, your best course of action is to have them made, as Kube did by welding OR as I have done, with more work and effort, by turning and internal threading two pieces together. I wish I had more encouraging info to pass along....Good luck.