If your PA is broadband, you need to measure the performance at several frequencies. If it is a routine narrowband (e.g., like a radio transmitter), then you need only one frequency to measure.
I suggest use the same connection type as provided in your PA. I presume it will have same connector types for input and output. If the RF output is not properly terminated, you may expect serious problems. However, you do not need much power to do spectrum analysis and can safely divert a fraction of the output via a SMA connection. 6.78MHz falls in SW band range.
I do not know signal generators that can supply 1W power at 6.78MHz.