By saying that the 8620 is not heat treated, do you mean that it is normalized instead of quench & tempered? If normalized (or possibly annealed), the 8620 will probably be comparatively soft. However, it can still form a fairly hard HAZ. So preheat to at least 250F. The weld only has to be as strong as the weaker material (assuming there is adequate cross sectional area of the weld) for the joint to be full strength. Using the lower strenght matial also reduces the risk of cracking because the residual stresses will be lower.