* S&P 500 coming off all-time closing high, targeting record intraday high 1,576.09.
* Further gains may be tough to come by
* ISM manufacturing data on tap after market opens
* Dow down 17 pts, S&P down 1.1 pt, Nasdaq up 0.25 pt
NEW YORK, April 1 (Reuters) - U.S. stock index futures were flat on Monday, with investors looking for reasons to keep pushing shares higher after the S&P 500 closed at a new all-time high.
* Equities have been on a tear in 2013, with stocks jumping 10 percent in the first quarter, extending their monthly string of gains to five. With the advance, the S&P is only a few points away from its all-time intraday high level of 1,576.09.
* While market momentum has been positive, many investors have been calling for a near-term pullback, saying shares have become over-extended.
* Investors will also look ahead to the U.S. Institute for Supply Management's March reading on manufacturing activity at 10 a.m. (1400 GMT), with the main index expected to come in at 54.2, same as the prior month. February construction spending, which is also scheduled for release at 10 a.m., is seen rising 1 percent, compared with a 2.1 percent drop in January.
* Economic data has been mixed recently, but strong reads may not be a positive catalyst for markets if investors view the economy as so strong that the Federal Reserve will reduce its bond-buying program, which has been credited with boosting equity prices.
* This is a busy week for economic data, with the ISM services sector report on tap for Wednesday and the closely watched non-farm payrolls report on Friday.
* S&P 500 futures fell 1.1 point and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures fell 17 points and Nasdaq 100 futures rose 0.25 points.
* Volatility may be low with European markets closed for the Easter holiday. Concerns over the region's debt, especially with respect to Cyprus, have been a major source of market uncertainty in recent weeks.
* The uncertainty over Cyprus has contributed to stocks trading in a tight range, with the S&P 500 within 10 points of its previous all-time closing high for 13 sessions before breaking through.
* The S&P's 10 percent surge in the first quarter has very bullish implications. An analysis by Ryan Detrick, senior technical strategist at Schaeffer's Investment Research in Cincinnati, showed the S&P 500 has risen in the three first months of the year nine times in the past 30 years, and in each case, it has posted gains for the year.
* The average yearly gain after such a start, the data showed, was 17.56 percent. An advance like that would leave the S&P 500 at about 1,676 at the end of this year.
* In company news, Exxon Mobil Corp continued cleanup of a pipeline spill that spewed thousands of barrels of heavy Canadian crude in Arkansas.
* Dell Inc warned that it would be dangerous to take on a lot of debt and remain a public company given its worsening profit outlook, in a sign that it views proposals from Blackstone Group LP and billionaire investor Carl Icahn as fraught with risk. The comments came on Friday, which was a holiday for U.S. markets.