Low engine temp

First Check the heater valve is working and opening to allow the hot coolant through.

Second check to see if the heater matrix (a small radiator) is blocked.

Either of these could be the cause or even a blocked pipe. It could also be a failing or failed water pump, but it should have shown signs of overheating...

I have assumed that the new thermostst is functioning correctly - perhaps testing it would be a good idea...

I noticed that you mentioned Dexcool in your comment to solar Mike's answer. There is a known issue with Dexcool turning to a kind of sludge or mud after many years of use in a cooling system. This could easily clogged up something like a radiator core. Just do a Google search for Dexcool sludge or Dexcool mud. You may want to check your cooling system to see if it needs to be flushed here is a good Guide to the subject :

http://www.sancarlosradiator.com/VoltageDrop/flush.htm