LOS ANGELES — Foster the People don't want to reveal too much about their performance at Sunday's 54th Grammy Awards, where they'll share the stage with the surviving members of the Beach Boys (together for the first time in more than 20 years) and Maroon 5. But they will tell you what song they won't be playing: their signature hit, "Pumped Up Kicks."

"I think my head would explode if [the Beach Boys] were playing 'Pumped Up Kicks' with us," frontman Mark Foster told MTV News during Grammy rehearsals Thursday. "I would probably have to say 'No, no, no. That's unacceptable.' No, we're going to be playing with them."

Both FTP and Maroon 5 will honor the reunited Boys — Brian Wilson, Mike Love, Al Jardine, Bruce Johnston and early member David Marks — by singing a medley of their classics, before teaming up with the surf (and psych) legends for a triumphant final take on the seminal "Good Vibrations." And though all three bands have been rehearsing the performance for two days now, Foster is still having a difficult time believing it's actually happening.

"We walked into the rehearsal room and met all of the Beach Boys for the first time yesterday, and it was a trip for all of us," he said. "We grew up listening to their music since we were little kids — I probably wouldn't be in music today if it wasn't for them. So, to see them all in the same room together, and then playing with them, it's wild. It's a dream come true."

So much so that, in a year that saw FTP not only score a certified smash with "Kicks," but tour the world and earn a pair of Grammy nominations, Sunday's performance unquestionably will be the highlight of their young career.

"I think out of everything we've done so far, this is the top for us. This is the icing on the cake," Foster explained. "We were talking about it yesterday: Exactly a year ago, we were playing in front of 250 people in L.A., and so much has changed since then. It's all kind of a blur looking back on it, but we were reflecting on it yesterday ... our album came out five, six months ago, and now we're playing with the Beach Boys at the Grammys. Things have changed pretty quickly."

So, win or lose on Grammy night (and, for the record, Foster seems to think his band is going to get creamed in both the Best Alternative Music Album and Best Pop Duo/Group Performance categories), Foster the People are going to be just fine. After all, they're playing with the freaking Beach Boys. And it doesn't get much better than that.

"I think people that are tuning in are going to see a piece of history. This is the first time the Beach Boys have played together in over 20 years," Foster said. "They're one of the best bands of all time. Brian Wilson has left such a mark on musical history, to be able to see him up there with the other guys doing what they do, it's amazing."