Mass is specified in Kilograms (kg). VPP uses SI units everywhere unless explicitly specified otherwise. You must convert your mass magnitudes (170, 350) to kilograms and use the resulting values in the Rigidbody.
Jpickup's mass is 1500 kg. This is within the range of its vehicle class, which goes from 1200 to 1700 kg (source).
When the mass is subject to gravity it becomes weight. At Earth's gravity at sea level 1 kg of mass becomes 1 kg of weight, and that's the correspondence in VPP. In reality weight varies due to small changes in Earth's gravity based on the location and the altitude.