slow starter at wits end

hello, i am having issues trying to start my rebuilt engine. its a 383 833 combo, when i try and turn the engine over its very slow/almost not wanting to even turn. I turned the engine over by hand when i was setting the timing and didnt notice any issues. i had an extra starter that i swapped out and its the same thing, i even jumped the starter relay and nothing changed. any idea what else could be causing this?

Lets start with battery. What is the voltage prior to and the during crancking?