This storefront on Third Avenue South will, if all goes to plan, be transformed this summer into the Johnny Cash Museum.

Since the day Johnny Cash tossed him a harmonica from the stage in Denver, Colo., Bill Miller has been hooked.

When he snagged that harmonica, Miller began a life as a collectibles entrepreneur and self-described “greatest fan” of the Man in Black.

Fans will soon get a chance to view the collection when Miller opens his Johnny Cash Museum at 119 Third Ave. S. sometime this summer.

“We have every confidence that Johnny will bring them in,” Miller says from his home base of Corona, Calif., where the exhibits featuring his collection are being constructed before being trucked to Music City.

“It will be an accurate portrayal,” he says. “If you take a group of school kids through, if they spend an hour in the museum, they would be able to do an accurate report that would trace his essence.”

The museum isn’t designed for schoolchildren, but for Cash fans of all stripes. But it’s not difficult to understand Miller using the youngsters for his illustration. After all, he was just 12 when his dad drove from New Mexico to Denver that first Cash concert.

“He was everywhere,” Miller explains, as he describes how he developed his passion. “There was Folsom, there was the TV program. That really let me know he was a fascinating person, far beyond the music. I really fell in love with what Johnny Cash was doing and what he represented.”

That devotion is what led his pop to drive him about 500 miles to catch the show in Denver. “I went down to the stage with my Brownie Instamatic,” he says. “(Cash) looked straight in my eyes and tossed me the harmonica he’d been playing.”

That harmonica won’t be at the museum. It hangs on Miller’s wall. But what will be there are things he’s pulled together in the years since the harmonica toss.

In addition to becoming the fan club teen editor, Miller found himself at countless Cash shows. “And I’d find my way backstage whether I had a pass or not.”

As he grew up, he became a friend of Cash’s and the collection continued, with some of the merchandise and collectibles being offered at JohnnyCash.com, the official online home of Johnny and June Carter Cash. Everything from T-shirts to music to valuable autographed items are for sale at the website and subject to the approval of John Carter Cash and Lou Robin, who strive to maintain Johnny’s image.

Cash himself “gave his blessing” when Miller said he wanted to start the website, which also is tied in with johnnycashradio.com and its round-the-clock musical tribute to Cash.

“We had been looking at doing this (opening a museum) for a number of years,” Miller says. “I have always proceeded with the thought that if and when we do this … we won’t go forward until the collection has reached the point where the artifacts illustrate Cash’s life, all the way till the time he passed.”

Other museums to country stars have failed. But Miller, who just recently, with wife Shannon, hosted “The Ultimate Johnny Cash Cruise,” is confident this one, which also has mementoes and the support of the estate, will succeed.

“It’s a natural evolution,” he says. “It’s the result of a new generation discovering him …. The authenticity that Cash brought to his music has crossed over, new generations embrace him.

“Johnny Cash has never gone out of style.”