Hi, I purchased FSX Gold to copy the SDK from disk 1 to use with Steam Edition since Steam Edition doesn't contain the full SDK, I know its on both disk 1 and the acceleration disk, don't know whether to copy the Sdk from the disk and overwrite the folder that is in steam edition or not.
Install the P3D SDK v1.4 into the default location... BUT,... if you have installed the Full FSX Gold Edition, along with the SDK for such (according to the guidelines within the above link), then you will not need to install the P3D SDK. You should be able to use scenery that you have created for both the FSE (Steam) and FSX.
Maybe you can trick your PC into "thinking" it has FSX box instead of SE installed by editing some registry entry or something which allows to install the normal SDK. I don't know why the SDK is dependent on having the sim installed, it makes no sense whatsoever.