Wall Street executives reacted cautiously to President Reagan's proposal to slash taxes and federal spending.

Most said they were heartened in the main by the Reagan plan, the details of which have been leaked to the press for the last several days. But many said they were worried about the new administration's ability to guide its first major initiative through Congress and were concerned that the president backed off some of his earlier proposals to cut taxes equally at all income levels.

A few executives said they felt let down because Reagan did not cut taxes as much, in percentage terms, for upper-income taxpayers as for those in the lower brackets, although in dollar terms wealthier taxpayers will benefit more than others.

"It was supposed to be an incentive-type tax cut," said one banker. "It's lost a lot of that, especially because there is no change in the maximum tax on earned income," which is 50 percent.

"The tax rates, even though they are lower for upper-income taxpayers, actually result in a greater rate of change in taxation as an individual moves from one tax level to another. That's a disincentive," said the head of a medium-sized brokerage firm.

But although some purists feel Reagan should have gone further in his tax-cutting, many echoed the sentiments of James Balog, senior executive vice president of Drexel Burnham Lambert Inc., a big Wall Street brokerage firm.

"It's not perhaps a perfect fulfillment of his campaign promises, but it's a courageous step that fulfills them in a large degree. There's wisdom there in saying that federal tax policy cannot be turned overnight. It's quite clear that there's a nice balance between the Kemp-Roth proposals and fiscal realities," Balog said.

While Balog and others feell the Reagan proposal has a better chance of congressional approval than the broader tax plan he campaigned on, there is still apprehension among many Wall Street officials that even the tax plan as proposed will have severe problems in the Democratic-controlled House of Representatives and that economic events beyond the administration's control could sandbag the ambitious budget cuts and reduced deficits in the proposal.

"It's the implementation that keeps us concerned," said an economist for a major bank. "We saw it last year when President Carter forecast a $15 billion deficit and a 9 percent increase in spending. That turned into a 14 percent increase in spending because of factors beyond his control."

"In general it appears to be a constructive program, although there are some question marks and wishful thinking as to what might have been political abilities to get it through Congress," said Jerry Hinkle, manager of trading for a big brokerage firm, Sanford C. Bernstein & Co.

Although the administration offered an ambitious program of budget cuts and tax cuts and projects a federal budget surplus of $500 million by fiscal 1984, Treasury Secretary Donald Reagan told reporters he could not agree an optimistic forecast on short term interest rates.

Reagan said he does not see rates falling below the double-digit level for some time.