Losing coolant extremely fast

[CSA026]

I just got done with a 5.3 rebuild and put it back in the truck (an '05 Silverado 4×2). Once I got everything hooked up and looking good I hooked the battery up and turned it over. It started right up like it had never been out and gone through. Excellent!..BUT...As I go to top off the antifreeze I noticed it was a little lower than when I started the truck. I look under it and coolant is just pouring out from close to the back of the transmission. It's losing coolant so fast that it looks like I'm dumping it straight from the jug onto the top of the transmission. I've checked the freeze plugs on the back of the heads, the hole where the temperature sending unit would be if the head was on the driver side, the coolant crossovers, and the head gaskets as well as I can without removing the heads.
Can someone tell me something I may be overlooking with this mysterious leak, please? It would be greatly appreciated.

[SMWS6TA]

Check the freeze plugs on the heads