what is the measurment of the carb mounting flange of a 4bbl squarebore?

I have to make a carb plenum for my '78 Buick turbo as they are no longer available, I will make it out of exhaust pipe and some thick sheet steel but I need to know how many " square the base of a squarebore ( will run a Holley Blower carb ) 4bbl is so I can determin what size exhaust tubing I need. I would measure myself but I don't have a squarebore or even spreadbore carb to check. I know I will increase the curb appeal of my Falcon by turbocharging it and I know I could ask a little more for it after turbocharging it, so now that I have a solid job again, I can start thinking about this again.

Pick up a gasket at a parts house...

that will work, then I'll save it for when I get everything togther. I never thought of using a gasket for the measurement.