How would i set a Part's CFrame to be connected at 2 ends?

So basically i'm trying to make a house Building system but I can't get the CFrame to be at the ends but it is in between the Markers

!enter image description here

part.Size = Vector3.new(.5, 20, math.abs((Marker.Position.Z - OldMarker.Position.Z)))

part.CFrame = CFrame.new(Midpoint(OldMarker.CFrame.X, Marker.CFrame.X), part.Position.Y, Midpoint(OldMarker.CFrame.Z, Marker.CFrame.Z))

--* CFrame.new(part.Position, Marker.CFrame.Position)

And when i used

part.CFrame = part.CFrame * CFrame.new(part.Position, Marker.CFrame.Position)

but the position Y coordinate = -10000000

Soooo yeaa....