Sorry, another quick question.... after installing the ROM and GAPPS should you clear cache etc immediately before rebooting?
I don't know if it is mandatory or if it helps, but I always clear cache and dalvik before rebooting. I also clear it before flashing GAPPS.
I haven't timed the boot, and it has been a while since I upgraded, but I think 5 minutes should be sufficient for the first boot. I'd wait 10 minutes to be sure. If you still can't boot, try flashing again with Pico GAPPS.