Not all GT40 heads came on 302's... so you may or may not need to have the bolt holes drilled out. GT40p heads only came on 302's and would require drilling. Those heads are cheaper, more readily available and generally make more power for this application, so you may want to look into those instead.
As mentioned above, valvetrain parts will indeed swap over from stock heads, but do get a shim kit in order to dial in lifter preload. If you cant get by with .090" of shims (or less) then you may want to consider new (shorter) pushrods.
Agreed that a cam and intake upgrade at the same time would give you the best bang for your buck. Dont forget to install a new double roller timing set as well.