1987 Cadillac Deville, replace power brake booster

I have a 1987 DeVille and I am in the process of replacing the power brake booster and master cylinder. I have removed the master cylinder and three(3) of the four bolts holding the P/B booster to the firewall. I can not get to the upper left bolt to remove the nut holding this last screw.

Does anyone have any suggestions as to how I can get to this last bolt/nut? Is there a special tool(s) that I can buy? What I really need is a 3/8" drive, 10" flexible shaft extension tool for a 3/8" socket, but I've never seen such a tool.

Help!

Help!

JoeJ