That's a great site you have going on the build of your '67 Camaro. I'll surely check the site and your progress often.
When I read about your Muncie popping out of third gear upon decel, I thought I should mention that you can probably fix that for good with a set of "torque lock" (some say "hook lock") sliders: http://www.thegearbox.org/catalog/item/3864782/3547824.htm
I used to have the same problem in my old vintage Trans-Am Camaro (Muncie M21) until I switched to "torque lock" sliders.
It seems from your site that you're not sure about sticking with the Muncie transmission, but I think they're a great piece. I've been vintage racing my Camaro for 11 years now, and have only done a couple rebuilds to the transmission in all that time. I own or have driven other cars with well built T10s or Ford Top-Loaders, and the Muncie in my Camaro has a better shift feel than any of them.