New battery..... dead?

[White70C20]

Hey all,
A few days before I went home for Christmas, the battery in my green 1970 C20 died, so I replaced it. Then about three days later I parked it at my uncle's farm for a month while I went back home. When I came back to Kansas it started right up. I didn't have any problems, until yesterday. When I started it yesterday afternoon I noticed it took a little longer than usual to start, but didn't think too much of it. Then when I was leaving the auto parts store it really took a bit to crank it over. When I got back, I shut it off and tried to restart it. It cranked very slowly for about three to four revs and then just clicked. It did that every time I tried for about 10 min. afterwards. I gave up and called it a day.

Sorry for the long post, anyone got any ideas? I really need to get this fixed because I'm trying to sell the truck. Thanks.