Vince -
In addition to what you already know, you will need the GTO interior door badges. GTO also offered a "fish scale" dash, around the speedo, but also had woodgrain. Interior-ise, that is about it.
You can add the speedo with tach & clock, but was only optional. You would then need the console gauge package.
Exterior, ENTIRE front clips from the Omega, Apollo and Ventura will work. The GTO grilles may be a bit hard to locate, as they are different than the Ventura, the GTO has the fog lights in them, Ventura does not.
Then you have the functional shaker & hood to consider, again a bit tough to find, but possible.
A Poncho 455 will bolt in, as the 305, 350, 400 & 455 are the same block size (not bore, but actual exterior look). You will need a trans from a buick pontiac or olds as the bellhousing is different than that of a Chevy.
There is currently a 74 GTO builder for sale out on E-bay that may interest you...then you would have the real mckoy!
Good luck!