I have 3- 400's here at present. One is in my 78 Bronco and it has the dipstick on the driver's side going into the middle back of the pan to a rear sump. The engine that came out of the Bronco was the same. I also have a '71 400 that came out of a car and it has the dipstick on the front of the engine on the passenger side in the location you describe. That engine was rebuilt and has a truck rear sump pan on it and the hole for the dipstick in the pan is blocked off. The PO of the engine did that.

It seems the 351m/400's from autos used the front location and the trucks the side location for the dip stick.