1965 small block A904 leak

I'm pretty sure I located the leak in between the extension housing and the transmission case. I started taking apart the slant 6 transmission that I still have laying around and noticed that I should be able to get the extension housing off even with the engine and transmission in the car. I will have to remove the (tti dual) exhaust, which I was trying to avoid, because it was difficult enough to adjust so it wouldn't rub on the body etc. I guess I could just mark the joints so I can put it back just like it is. I haven't been able to find a "new" gasket, but I found a NOS one for 12 bucks on ebay. I absolutely love the old Mopar packaging by the way. However, the material is very thin and I wonder if I should just use it as a template and cut my own gasket out of a better/ thicker material?