please help!! truck wont start

[nac593]

1969. new everything. new crate motor,hei dist., alttenator,battery....I have put 6000 miles on it since putting the truck together. Everything has been fine. Starts right up USUALLY....Yesterday morning I jumped in and went to start it. It was having a real hard time cranking. Voltmeter showed 8 volts while cranking. I thought the battery was bad so I had it checked at the store and they said it was ok. I charged it , put it in, tried to start it and same thing. 8 volts and struggling to crank. I went and bought a brand new battery and same thing. 8 volts while cranking and struggling. It only cranks a few seconds and the new battery already seems drained. I thought with a new battery it would crank fast but it still barely cranks. Everything has been fine since I put it together about 1 year ago. More info...altenator was showing 18 volts...I replaced the altenator and it showed about 14 volts...drove it about 2 days and then this happened.. PLEASE HELP!!!!!!