congrats on the successful transformation! I am not personally familiar with the bird-voiced treefrog since I'm not in their range but in reading up on it on of the ways to tell them apart is the "flash" colors under the hind legs. Gray Treefrogs have orange while Bird-voiced Treefrogs have lime. I don't know if that holds up with metamorphs or not. The calls are also very different but that won't help you for quite some time

here's a decent picture I took of the flash colors of an adult gray treefrog:
http://www.uvm.edu/~kvbriggs/pictures/GrayTreefrog/rn_img_0993.jpg

Again, I've never dealt with Bird-voiced Treefrogs but I have seen a number of metamorph gray's and it just doesn't look right to me -- that doesn't mean anything though. Here's a picture of a metamorph gray treefrog from earlier this summer