Generally, the only time a preheat is used is when two rather drastically thicknesses are being joined, like your case. The 80mm thick plate, depending on the overall size, can cause a "heat sink" that has a slight chance of causing a rapid cooling rate that could possibly initiate a solifidication crack in the weld bead. It is possible, but not probable. The most important thing to try to achieve is a somewhat even cooling down rate on both parts. It might not hurt to warm both parts up to around 150-200F in the vicinity of the weld just to slow down any rapid cooling rates. If you will do this, there will be minimal weld stresses. A solution anneal is not recommended as a form of heat treatment for weld stress. That is generally used to eliminate any intermetallic phases that might have occured during the welding process. Both of your parts are of a low carbon grade, so any phases are unlikely. Warm both parts up and use a relatively high heat input and allow both parts to cool as evenly as you can, and you should have no problems.