With the economy down and needs up for the homeless, the hungry and the elderly, donations to South Florida churches and other religious institutions are straining to keep up with soaring needs, leaders say. At the Miami Archdiocese, collection-plate revenues are steady, but assessments that individual parishes pay are slow in coming or are down, and needs are up sharply, resulting in the layoff of 49 of the 182 staff members at its Pastoral Center on Biscayne Boulevard, said spokeswoman Mary Ross Agosta. In a letter to parishioners, Archbishop John C. Favalora said: "Each year, a greater number of parishes and programs are seeking our financial help, and, therefore, we must prioritize. We can only work with what we have."