Just to be sure you should still seperate the motor from the trans to see what happens.......or not.

 

Electrical gremlins can be a beitch.. I went thru a similar issue on my Mustang years ago. It wouldn't crank hot. Amp test showed the starter drawing 700amps!! Replaced it and nothing changed. Ended up being a bad ground cable that still looks brand new but had gotten a lot of internal resistance. Glad you figured it out.

If it would have locked up, would have been a good time for a 6.4L upgrade

 

Glad you got it figured out