"This gonna be a good series, but I got us in 6. And if it goes 7, we’ll win," 40 says without hesitation.

Of course, the Warriors, who entered the 4th up 12, were outscored 40-16 in the final quarter … on the back of Al Horford. The vet scored 26 points in the game … with 11 coming in the last 5 minutes of the game. Boston won 120-108 over GS.

"We had them. 4th quarter come, them boys would not stop hitting them 3s."

But, the rap legend says that's unlikely to continue ... and that's why GS should still win the series.

"As long as they shooting like that throughout the whole series we gonna be straight. That was very rare. They was hitting and we was bricking.”

We asked 40 if game 2 was a must win for the GSW … 'cause going to Boston down 2-0 isn’t far less than ideal.

"I think [game 2's] very important. The home team advantage is always the best. As long as we show support, we get louder in there, we show them we’re right there with 'em. I think we’ll win [game 2], and I think we'll win a couple of games on the road. I think we’re gonna surprise everybody. We just had a bad 4th quarter man."

And, if the Warriors need any more motivation to win themselves rings … 40 says he’s going to hook up the MVP with a bunch of liquor, wine and champagne from his Earl Stevens Selections.

Check out the clip -- we talk to E-40 about a bunch more basketball-related topics ... from the cost of his courtside tix (yes, 40 pays!) to celebs like Jay-Z showing up to watch the Celtics and Warriors hoop.