Basically, what this does is returns a CFrame value from the given CFrame parameter in WorldSpace.
So, we can use this to achieve your request:
(Make sure your Model has a primary part set to it)
local Model = workspace.Model --Set this to the model you want ofc
local Part = game.Players.LocalPlayer.Character.Head --This is the part to go behind. Will be using the player's hide like you did.
local Offset = CFrame.new(0,0,5) --This will be the offset of the position from the Head to the model
local NewModelCFrame = Part.CFrame:ToWorldSpace(Offset) --This will get the CFrame of the position behind the head with the offset we gave it.
Model:SetPrimaryPartCFrame(NewModelCFrame) --This will put the Model behind the head, 5 studs away.
The above script will also make the model have the same orientation of the head. If you dont want this, just use :PointToWorldSpace() instead of :ToWorldSpace() which returns a Vector3 value instead of a CFrame value.