Will a 1979 4.2 jeep engine fit in 1987 Jeep Wrangler?

No, a 1979 Jeep 4.2L engine (AMC 258) will not be a direct fit into a 1987 Jeep Wrangler (YJ). While both are 4.2L engines, there are significant differences in the mounting points, accessories (like the alternator and power steering pump), and the engine's overall design between the two generations. You would need extensive modifications to make it work, and it's generally not considered a practical swap. It's far easier and more cost-effective to use a 1987 or later 4.2L.