Will the oil pan of a 307 or 350 fit on a 403 and if so what year range? I own a 78 403

 

All Olds oil pans are interchangeable. Does not matter what size, 260 thru 455.

 

All 1964-90 Olds V8 oil pans interchange, but they are not all the same. Setting aside the Toro pans, the SBO and BBO pans are different. The BBO pans have a pair of "bumps" just forward of the sump. The SBO pans do not. Of course, the O.P. is asking about 307 to 403, which are both small blocks, so no problems there.