How can I strengthen the floor above a crawl space in pier and beam construction?
The new support structure with a beam that you are proposing sounds fine. 

If you are concerned, then overdo it by doubling the materials for the beam, or placing a steel L-rod 3/8" thick around the beam. Using an L-rod will reduce the number of bolts that you need to put through it into the beam, and will add structural support due to the angle.

The more important thing to remember here is what the pier block sits on. If you have bedrock or a similar rocky aggregate under the house, then you may be able to safely rely on it not shifting or changing over time.  

If it is just soil, you can expect that you will get some compacting due to the weight of the heater. Even if you are able to bring in a tamper and pack the soil, you can still expect that over time it will change through natural cycles in the ground.  Instead, place the broad flagstones under the peers to distribute the weight. 

Make sure that the flagstones are thick enough that the weight will not crack them – since the breadth of the flagstone is being used to distribute that weight across the soft soil. Do not use metal, as it will rust and deteriorate.