President Reagan signed a law Thursday swapping nearly 40,000 acres of federal land in Nevada for 4,600 acres of Florida wetlands owned by Aerojet-General Corp. The measure, "Nevada-Florida Land Exchange Authorization Act of 1988," is an unusual arrangement, but Reagan noted it offers advantages to both the government and Aerojet-General. In addition to acquiring 38,840 acres of public land in Nevada, the company will be able to lease an additional 14,000 for 99 years, Reagan noted in a statement. The land will be used for rocket building and testing facilities and will "expand significantly the tax base" for the area and "also provide employment opportunities," he said.