Y'all were right: it was the timing. When I got everything off the front of the engine, I could see that both cams were advanced about half dozen teeth relative to the crank. I have no idea how I did that. When we put it together the first time, we got the lines on the belt on the marks on the sprockets, and stood back and admired it, and looked twice...and somehow we got it wrong.
So, we reinstalled the belt, and got the lines on the belt on the marks on the sprockets, and looked twice, and somehow we got it right, because this time it started right up.
Thanks for your help!