Question: The 1967 Trans-American Championship was the second running of the Sports Car Club of America's Trans-Am Series, which year would also mark the debut of Mercury, with Dan Gurney winning at Green Valley for the company in its new Cougar, Gurney also reached racing's highest levels starting in which year?

Context paragraphs:
History of the Trans-Am Series: The 'Trans-Am series' is an automobile racing series that was created in 1966 by Sports Car Club of America (SCCA) President John Bishop.  Originally known as the Trans-American Sedan Championship, the name was changed to the Trans-American Championship for 1967 and henceforth.  The series has in fact gone by a variety of different names through the years (too many to list), some linked to sponsors, some not.  It has evolved over time from its original format as a Manufacturers' Championship series for modified passenger sedans and Coupés to its current form as a Drivers' / Manufacturers' Championship Series that is open to GT style racecars.  Champion drivers have been officially recognized, and Drivers' Championship(s) have been awarded since the 1972 season.

1967 Trans-American Sedan Championship: The 1967 Trans-American Championship was the second running of the Sports Car Club of America's Trans-Am Series.  After the dominance of Alfa Romeo in the under 2000cc class in 1966, Porsche would rise to prominence, starting a dynasty that would last for several years.  1967 would also mark the debut of Mercury, with Dan Gurney winning at Green Valley for the company in its new Cougar.  David Pearson would also win in a Cougar at Riverside later that year.  Ford and Mercury would both end the year strongly, with four wins apiece.  Mark Donohue would provide the first win in the series for Chevrolet at Marlboro Speedway.  He would go on to win at Stardust and Pacific Raceways, igniting the Ford vs Chevy rivalry that made the series legendary.  Ford and Porsche won the manufacturers' championships.

Dan Gurney: Daniel Sexton Gurney (born April 13, 1931) is a retired American racing driver, race car constructor, and team owner who reached racing's highest levels starting in 1958.

1972 Trans-American Sedan Championship: The 1972 Trans-American Championship was the seventh running of the Sports Car Club of America's Trans-Am Series.  The first seven rounds used split classes, while the last four classes had the Under 2500cc class only.  Milt Minter brought Pontiac its first win, at Mid-Ohio.  George Follmer and American Motors won the over 2.5L division, while John Morton led Datsun to the championship title of the "Two-Five Challenge".

1958 United States Grand Prix for Sports Cars: The 1958 United States Grand Prix for Sports Cars was a sports car race held at Riverside International Raceway on October 12, 1958.  It was the fourth and final round of the 1958 USAC Road Racing Championship season, the seventeenth round of the Sports Car Club of America's Pacific Coast Championship, the second running of the Riverside Grand Prix, and the first post-World War II running of the United States Grand Prix.  The race was held over 62 laps of Riverside's 3.3 mi circuit, for a total of 203.1 mi .  Chuck Daigh won the race overall, driving one of Lance Reventlow's Scarab-Chevrolets.  The race is also noteworthy in the annals of international racing, as the strong second-place finish by a local driver named Dan Gurney earned him a test drive in a factory Ferrari Formula 1 car, effectively launching the Californian's legendary racing career.

1969 Trans-American Sedan Championship: The 1969 Trans-American Championship was the fourth running of the Sports Car Club of America's Trans-Am Series.  The championship was open to SCCA Sedan category cars competing in Over 2 litre and Under 2 liter classes.  1969 marked the end of the use of co-drivers in the Trans-Am Championship, as most of the races were between 2.5 and 3 hours.  The Over 2 liter and Under 2 liter Manufacturers' titles were won by Chevrolet (Due in large part to Mark Donohue's 6 wins) and Porsche (Due in large part to Peter Gregg's 7 wins) respectively.

2001 Trans-Am Series: The 2001 Trans-Am Series was the 36th season of the Sports Car Club of America's Trans-Am Series.  The victory at Portland would mark Dodge's final Trans Am win until the 2012 Trans-Am Series.

2000 Trans-Am Series: The 2000 Trans-Am Series was the 35th season of the Sports Car Club of America's Trans-Am Series.  2000 marked the end of the "American muscle revival" era that had begun in 1989, with Italian manufacturer Qvale winning the championship.  It would also mark the rise of Rocketsports Racing's dominance using Jaguar XKRs, which would continue until the series dissolved in 2006, after which team owner Paul Gentilozzi would switch to the American Le Mans Series.  The season also marked the final victory for Pontiac in Trans Am, with a win at Texas.

1968 Trans-American Sedan Championship: The 1968 Trans-American Championship was the third running of the Sports Car Club of America's Trans-Am Series.  1968 marked the addition of the 12 Hours of Sebring and the 24 Hours of Daytona, the only year that the Trans-Am Series featured those races.  The season marked the first time the series ever left the United States, as the race at Mont-Tremblant brought Trans-Am into Quebec.  Chevrolet (Thanks to Mark Donohue's unprecedented 8 race winning streak, and Chevrolet winning 10 out of 13 races) and Porsche (Thanks to Tony Adamowicz' 5 race winning streak and Porsche's 8 wins in a row) won the manufacturers' championships.

1971 Trans-American Sedan Championship: The 1971 Trans-American Championship was the sixth running of the Sports Car Club of America's Trans-Am Series.  The format was altered to an over 2500cc class and an under 2500cc class, up 500cc from past years.  All races used split classes except where noted.  The year marked the rise of Datsun as a competitive brand, with the Japanese company winning several races throughout the season.  American Motors, led by Mark Donohue's 7 wins, and Datsun, led by John Morton's 6 wins, won the manufacturers' championships.

Supporting paragraphs: 1967 Trans-American Sedan Championship Dan Gurney
Answer: 1958