Thanks for the offer, Greg. I'm fixed now. I got it figured out late last night, finally.
I think I must've mis-assembled the lower turret assembly somehow. The glass portion was just not sitting down on the notches at the sides like it was supposed to. The molded pivots on each side for the gun, the curved shield in front of the gunner, and the gunner's armored headrest were preventing the glass from resting evenly on the notches. I ended up grinding off the headrest, and grinding down the sides and front until the glass could be dropped into place correctly. Here's a pic. Ground areas in red, notch and tab align points in yellow.
Probably just ham-handed assembly by yours truly, but I guess that's the way it goes sometimes.
Chris