Your question, "what engine" is many months old so my reply is somewhat academic.........nevertheless..........
Six cyl flat head engines on a 4 tonne truck could be Continental F226 or F227 models. The only difference that I recall was that the head was:

F226, head was secured with nuts on studs that were in the block
F227, head was secured by hex. head capscrews that fitted into tapped holes in the block (There were similar differences between the F162 and F163 four cylinder engines engines)

Regards Tim B, Hampshire, England