Not enough information to provide any guidance regarding preheat or PWHT because you do not state thicknesses, application or code of construction. Basically, develop a welding procedure, qualify it, and follow code of construction requirements for weld preheat and PWHT.
All necessary heating and cooling requirements for thermal treatment regarding preheat, post weld heat treatment and NDT are in ASME Section VIII, Div 1. Review it carefully.
Based on your original post and unfamiliarity with ASME Section VIII Div 1 and Section IX, you need help from a welding engineer in person not in a forum.
Ditto metengr's comments, you need a proper welding engineer (I am currently available, nudge wink). Even then, many WEs have no experience fabricating such heavy plate (this sounds like it is a reactor vessel, possibly for refinery). One thing is for sure, do not skimp on weld consumable quality, and scan the tech spec fine print for requirements that are over and above Code. The quality, capability, and experience of the fabricator are vital to success.
One pitfall will be this: the standard 1/8 inch depth specified for back cladding (to match SS321) will NOT be sufficient to achieve equivalent corrosion resistance. Draftsmen seem not to know this, having copied this dimension from one drawing to the next ever since Moses. Unless you have a well controlled, low-dilution two-layer WPS, you will need to allow room for three layers of weld if you plan to avoid corrosion. Three layers will necessitate almost 1/4 inch depth of preparation. Otherwise, in grinding flush with the surrounding base metal, you will grind down into the second layer and possibly even the first (buttering) layer. In any event, welding a large project like this requires something automated and specialized for the task (for both the groove welds and the cladding).
I have butted heads with several WEs over the cladding question, but I have observed many shell courses for SS clad polymer reactors that developed internal rust strips just from sitting out in the weather.
Why Div. 1? Traditional practice for welding vessels of this thickness used Div. 2 to further decrease T. Why purchase clad plate? Corrosion resistant weld overlay has traditionally been used for materials this thick based on cost/delivery.
Was base metal subject to impact testing? If not, it should have been.
Recommend to strip back existing clad minimum of 3/8" to preclude dilution of SS into "B2" deposited filler metal when making the strength weld. Clad restoration should have first layer made with 309LCb second layer same or with 347.
