The House agreed yesterday to stretch the fiscal 1979 spending limits set last September by an extra $1.5 billion to help entice both liberals and conservatives into supporting a companion budget resolution for fiscal 1980.

The move came on a proposal by Rep. Paul Simon (D-III.) to finance a large grab-bag of new spending measures, from two Spruance-class destroyers ordered and later canceled by Iran to added outlays for the food stamp program.

The vote to approve the Simon measure was 235 to 177.

Approval of the Simon proposal was expected to clear the way for the House to begin work-possibly today-on a spending plan for fiscal 1980. Last week the senate amended its fiscal 1979 total and passed a fiscal 1980 budget resolution with outlays of $532.6 billion and a deficit of $29 billion.

House members have been working first on amendments to the fiscal 1979 budget ceilings they approved last autumn. The fiscal 1979 budget had to be altered because of changes in economic conditions.

If the lawmakers do not make any further changes in the spending targets for fiscal 1979, which ends Sept. 30, yesterday's House action would put overall layouts at $493.3 billion-up from a $487.5 billion spending ceiling set last September. The House Budget Committee had recommended a spending ceiling of $492.8 billion.

The Simon proposal was intended to help win votes for the fiscal 1980 budget resolution by offering both sides some of what they wanted for fiscal 1979.

The measure was approved after liberals in the fiscal 1979 budget for the countercyclinical revenue-sharing program, which expired last summer.

However, yesterday's action is not expected to save that program. The House Government Operations Committee has indicated it will not extend the program, which was designed as an antirecession measure.

The lawnmakers are far behind schedule in considering the budget resolution and probably will have to continue into next week before finally passing it. Under the 5-year-old congressional budget process, Congress must set tentative budget targets by May 15 each year.

These are revised the following September.

In a late evening vote, the House defeated a bid by Rep. Charles H. Wilson (D-Calif.) to increase defense spending for fiscal 1980 to $125.1 billion, $1.2 billion more than the Budget Committee recommended and $900 million less than President Carter requested in January.

The proposal was voted down 209 to 188, with virtually no debate. Conservatives are expected to make more serious effort to boost defense spending in floor action today.