I have a 69 Camaro and the front sub is rusted, will a sub from a 74 Buick Apollo fit? I was told it will fit a Nova. If so, will I have to change anything or modify?

Apparently yes.
Camaro, Nova, Apollo & Omega share the same subframes.

You will have to drill some holes for the bumper brackets
as they are probably different.
Copy them off your old subframe.

You might also have to snap off the '74 motor mount tabs- they hang off the rear of the main crossmember. Compare both frames and you'll see them. Also, if you are using a TH400 crossmember you'll have to drill mount holes for it.