IT'S TAX WEEK, and millions of Americans are scrambling and swearing and sweating. But for America's most famous tax outlaw, the sight of April 15 is no sweat. "Hey, I'm not singing the blues -- I'm not even thinking about taxes right now," says Willie Nelson, calling from aboard his tour bus, which is rolling somewhere through Virginia. "Things are back close to normal."

Of course, Nelson, who appears Tuesday at Capital Centre with the Highwaymen (Waylon Jennings, Kris Kristofferson and Johnny Cash), is famous for underplaying everything, from his desert-dry singing to his relaxed acting, so it's no surprise that he's almost comically casual about his highly publicized tax trauma.

In 1991, the IRS auctioned off all of Nelson's assets to recoup a portion of the $16.7 million he owed in penalties and interest from an ill-advised tax shelter (Nelson's friends organized and bought up much of the booty with low-ball bids, and held it for him so he'll eventually get it back). The contretemps also inspired one of his finest albums, a dark-night-of-the-soul set called "The I.R.S. Tapes -- Who'll Buy My Memories?," which was sold via an 800 number to help pay off his debt.

"We agreed to agree," says the singer-songwriter, who recently settled with the IRS to the tune of $16 million. "Of course, I still have to pay them a little here and a little there" -- this is Nelson's offhand way of referring to the $5.4 million he's promised to fork over to the government within the next five years. But somehow Nelson's triumph over his travails provides inspiration for Everyman -- hey, if Willie can get through it, so can we.

Tax time aside, April's a banner month for Nelson: His 60th birthday is on April 30, though a mix-up in the birth papers has him listed as both April 29 and 30. "A Taurus with two birthdays -- double trouble," he laughs. Nelson is celebrating with a two-hour CBS-TV special, "The Big 6-0." He's scheduled to sing "Graceland" with Paul Simon on "Saturday Night Live" in May.

And on April 24, he's off to Ames, Iowa, for Farm Aid Number 6, raising millions for this country's beleaguered farmers with a lineup that includes Neil Young, Paul Simon, John Mellencamp, John Conley, the Highwaymen "and maybe Dylan," he says. But the main reason to tune in this year is undoubtedly the unveiling of Nelson's rap song about the plight of the farmer. (You heard right. Rap. Be there.)

To top it all off, Nelson's just released a new album -- his 35th -- and it looks likely to be his biggest success since 1978's "Stardust," a collection of standards produced by R&B ace Booker T. Jones.

Produced by Don Was, who has engineered the comebacks of Bonnie Raitt and the B-52's among others, "Across the Borderline" finds Nelson in celebrity country, singing songs by Lyle Lovett, John Hiatt, Paul Simon and Bob Dylan (they collaborated by fax), and duets with Bonnie Raitt and Sinead O'Connor.

Mingling with rockers and popsters (and he's suggested a reggae album may be next!) is nothing startling for Nelson, a country troubador who has made a career out of restlessly crossing and re-crossing musical borders -- never a conventional country singer, he's recorded with everyone from Faron Young to Julio Iglesias. (And before he was a musician, he put in time as a pig farmer, disc jockey, door-to-door Bible salesman . . .)

At first glance, the 14 songs on "Across the Borderline" look like a name-dropping supersession, an obvious mass-market media ploy, a crass grab for a few fast-money pop and rock hits. But Nelson denies such commercial calculation. "You just can't predict these things," Nelson says. "I mean, who ever would have predicted that 'Stardust' was going to be my best-selling record?"

And anyway, the career-saving "Borderline" is perhaps Nelson's most personal album to date. Repeated playings reveal a cohesive and moving concept album about disillusionment, abandonment, the crumbling American Dream. When Nelson sings from Paul Simon's "An American Tune," "I've often felt forsaken, and certainly misused/Oh, but I'm alright, I'm alright, just weary to my bones . . .," it's hard not to think of the singer's many trials. Likewise, the unlikely pairing of Nelson and O'Connor on Gabriel's "Don't Give Up" brings to mind both Nelson's many heartaches and O'Connor's recent self-incurred woes, as both singers radiate vulnerability and resilience.

"All the songs, except the ones I wrote, were brought to the sessions by Don Was," Nelson says. "I never would have thought of recording some of these songs. Like 'Don't Give Up,' for instance. We recorded it in October, the day after that big Dylan tribute concert at Madison Square Garden. When it came time to record it, I was thinking of Sinead and her troubles -- after what she went through that night, being booed and all, it was amazing that she came into the studio the next day at all. She's pretty tough, all right," he says admiringly.

Anyway, along with the album and the current tour, Nelson stands to make up a heap of that outstanding tax money with multiple product endorsements, lending his name and grizzled countenance to ads for Wrangler Jeans and Taco Bell (the fast-food chain is offering specially priced cassettes with certain purchases); and copies of "Borderline" come with an insert hyping Jose Cuervo Tequila, with a mail-in offer for a free pair of "collectible" Willie Nelson signature shot glasses.

"But hey -- don't even start on me about selling out," Nelson says. "I've heard it all before, and my dues are paid -- in full. And anyway, when it comes to the tequila, I've invested a lot of money in their product over the years," Nelson says, and laughs. "I'm happy to get some of my money back."

WILLIE NELSON AND THE HIGHWAYMEN -- Appearing Tuesday at Capital Centre. Call 301/432-7328. To hear a Sound Bite from "Across the Borderline," call 202/334-9000 and press 8101.