[Phanx]

There is no "boss frame container".

https://github.com/tekkub/wow-ui-sou...ua#L2310-L2323

	-- Boss frames - need to move below buffs/debuffs if both right action bars are showing
	local numBossFrames = 0;
	for i = 1, MAX_BOSS_FRAMES do
		if ( _G["Boss"..i.."TargetFrame"]:IsShown() ) then
			numBossFrames = i;
		end
	end
	if ( numBossFrames > 0 ) then
		if ( rightActionBars > 1 ) then
			anchorY = min(anchorY, buffsAnchorY);
		end
		Boss1TargetFrame:SetPoint("TOPRIGHT", "MinimapCluster", "BOTTOMRIGHT", -(CONTAINER_OFFSET_X * 1.3) + 60, anchorY * 1.333);	-- by 1.333 because it's 0.75 scale
		anchorY = anchorY - (numBossFrames * (68 + BOSS_FRAME_CASTBAR_HEIGHT) + BOSS_FRAME_CASTBAR_HEIGHT);
	end

That said, as long as you're not trying to move it in combat, I don't see why a simple hook shouldn't work:

local moving
hooksecurefunc(Boss1TargetFrame, "SetPoint", function(self) -- don't care about args
	if moving or InCombatLockdown() then -- avoid infinite loops and blocked actions
		return print("Boss1TargetFrame:SetPoint", moving and "already moving" or "in combat")
	end
	moving = true
	print("Boss1TargetFrame:SetPoint moving...")

	self:ClearAllPoints()
	self:SetPoint("TOP", Minimap, "BOTTOM", 0, -70)

	print("Boss1TargetFrame:SetPoint done!")
	moving = nil
end)

If you still get the "not movable or resizable" error it would help to specify what exactly is on line 342 in your file...