New commercial mount for 2004R transmission

I'm totally sorry but you have to have a reference point. When you say, "6 1/2" or 8 1/2"" makes no sense. I'm not trying to be an @ss, I'm honestly trying to help. Your transmission tail shaft or output shaft should be at an angle that makes your engine and transmission 3 to 4 degrees down hill. If your car is on a lift the frame should be horizontal (you can check and make sure). When the factory installed the engine they made sure that the engine/trans were at 3-4 degrees down at the rear so that engine oil flows down and to the rear of the engine. To be honest the trans tunnel has nothing to do with it. The only thing that matters is "Engine/Trans angle".
Please don't beat me up, I've installed many engines and transmissions in many cars/hot rods etc.

I recently installed a 2004R GM transmission in my '69 Barracuda. All I cared about was the engine/trans angle (when the frame is level). NOT the distance from the tunnel to ANYTHING!! You can completely remove the tunnel and you would still have to get the engine at 3-4 degrees down hill (front to back).

Even 2 degrees down at the trans output shaft would work....

Hope this helps........
treblig