I have an AT&T SGH-I337. I successfully downgraded from NB1 to MK2, rooted, and installed SafeStrap. I had successfully booted to stock MK2 at this point.
I downloaded Deadly Venom Kitkat Magic Tool. I used SS to wipe all except external sd. Stock rom is active. Then queued 4 zips to install -> GE-SAFESTRAP-XXUFNBH-26.zip, GE-SLiM-F4ST-ADV-KK-26.zip, UPDATE-SuperSU-v1.93.zip, and AT&TNB1KernelModules.zip. I hit home -> reboot -> download. I used ODIN to flash AP, BL, and CP from Deadly Venom tool. I unchecked auto-reboot, flashed, hold power. Safestrap says 'Safestrap Disabled', then I get bootloop at the Samsung logo. Next I went back to download mode, tried to flash AP, BL, and CP again with auto-reboot enabled. Still stuck at Samsung logo.