starter issues

i just replaced the head gaskets on my 92 f250 idi and the first issue i had was that it would take forever to turn over and start unless the block heater was plugged in... now all of a sudden i go to start it and after the glow plugs cycle through i turn the key and hear a single click as if theres not enough amperage to kick over that starter... yet both batteries and starter were removed and tested and everything checked out perfect... it will push start no problem though...
at this point i am at a complete dead end on what could be the problem and i need this truck to get to work so any help would be great