If you put a new engine and transmission?

In place of an old one do you they have to be the same year? Or can I put a 95 4.6L V8 with a ford T-45 5 speed manual transmission of a different year? Thank you for your answers in advance. Stupid answers will be ignored. The 4.6L V8 is going in a 2003 ford mustang

Correction 97 4.6L V8 out of a 97 mustang.

Depends on where you are, and whether you are subject to periodic emissions inspections to maintain your registration. If you swap a different year motor into your car, it has to meet the Emissions standards for the Higher of the 2 years. A 1995 engine in a 2003 mustang must meet 2003 emissions specifications, so you need to make the 2003 catalysts and ecu electronics work with the '95 engine. It is typically easier to just use the correct engine.