Will a 4.9 engine fit into 95 cadillac sts that previously had northstar 4.6?

No, a 4.9L engine (likely referring to a Ford 4.9L inline-6) will almost certainly not fit into a 1995 Cadillac STS that originally had a 4.6L Northstar V8. The engine bays are designed around specific engine dimensions, and the different engine types (V8 vs. inline-6) and their sizes will result in significant incompatibility issues:

* Different Engine Block Design: The inline-6 and the V8 have completely different block designs, meaning the mounting points will not align.

* Size and Dimensions: Even if you could somehow get the engine to sit in the space, the 4.9L's dimensions (length, width, height) are unlikely to match the Northstar's, creating clearance problems with the firewall, body structure, and other components.

* Ancillary Components: The location and configuration of the accessories (alternator, power steering pump, etc.) are different, creating further incompatibility.

* Transmission: The transmissions are not interchangeable. The 4.9L would require a different transmission, which itself would create further mounting and compatibility challenges.

* Wiring Harness: The wiring harness for a 4.9L is completely different from the Northstar's, requiring extensive and complex rewiring.

Swapping an engine is a complex project even when engines are somewhat compatible; this swap is highly impractical and likely impossible without major, extensive, and likely prohibitively expensive fabrication work.