Yes, just about year 216/235/261 engine could have been swapped around.

Also, your 1941 truck might have come with a 1942 motor.

Or, you might have a 1942 truck that was sold as a 1941 truck.

What is a "pink slip"?

839910 was a 216 block casting code from 1942-1947

Find and post the head and block date casting codes.

What code is on your Vehicle Serial Number plate? (if you can find a plate)