Still a Z ,look at the VIN.
You can stick what you want between the wheel wells and you have a" Z " without the 302 . Can't put a 302 in what came with a 6 banger , glide and a 10 bolt and ever make it one.
Like was suggested ,if you're building it to go out and have fun or even have plans in the distant future to sell it , stick another engine in it and keep looking. Too many unknown variables with your car to suggest investing 10K+ in a 302 will immediately raise it to a level demanding top dollar. Plus a deal on one may surface.
Hate to say ,and am still on the fence, but probably will end up installing an early 327 I have , leaving the 302 on the stand . That was my thoughts years ago when it got a 400 for street/strip use. The final purpose should dictate the build.