What is a good cruising altitude for this type of plane? I cant find anything more than the service ceiling on the net.
It depends on if you want speed, economy or to survive the flight.
It will be fastest around 3000' but get better economy (and be slower) as you go up. Any thing much over 10,000 and you are going to start to run out of oxygen.
In the 1930's I don't know if they had ATC rules that stated VFR flights fly at this altitude or not. I would guess that did not come until later.
The best cruise is probably 4000' which would be a good mix of power and speed.
Ultimately for this plane it will be personal choice.
Dave