Alternative to US tax plan outlined by pair of unions

Two major unions, backed by the US Conference of Mayors, proposed a $24 billion economic stimulus package as an alternative to President Reagan's program.

The 1 million-member American Federation of State, County and Municipal Employees union and the American Federation of Teachers said the plan would ease sharply rising unemployment and could be financed by eliminating what they called ''giveaways'' for the rich in the President's tax cut plan.

The proposal includes:

* A block-grant program to repair streets, bridges, and water and sewer systems.

* Aid for railroads and mass transit systems, to be triggered anytime unemployment exceeds 7 percent.

* A $5 billion economic development bank offering loan guarantees and assistance to small businesses.

* A $5 billion program to retrain jobless workers for new jobs in growth industries.