1970, 351c 2bbl was starting just fine but now is extremely sluggish while trying to start.
Originally I tested the battery and it was low and not holding a charge. I replaced the battery and checked all my connections.
I didnt see anything out of the norm. Im thinking it might be the starter motor.
If you have a voltmeter, check the voltage present at the starter while cranking. With a healthy battery, my 70 351C sees about 10.5 VDC while cranking at the starter.
Also, your negative cable should be well grounded to the chassis at the voltage regulator (with clean bolt threads), and also where it connects to the block near the alternator.