From the question I am guessing you do not have access to PQR or WPS to work from which is not a good practice.
My thoughts on the subject that are sure to open the flood gates and you will most likely be inundated with more information than you can process in a short time.
The preheat is critical as you aware but a full PWHT could be detrimental because you may need the as welded properties which depending on what PWHT temp you use could lose.
I know of similar situations where a 600'F/8-16 hours Hydrogen bake out process was used versus a high temp PWHT.
I am not a Engineer or Metallurgist but I have preformed the above process in the field more than a couple times 20 years ago with out problems.