Measure the cly bore. The 390 has a 4.05 bore and the 352 a 4.00 bore. Also look at the casting numbers of the heads, intake, crank, rods etc....it's possible that all this might have been changed depending on how original the engine is suppose to be. The parts inside will give you a better idea what your looking at as cranks have different strokes and rods were different from engine to engine.The early blocks will have a two bolt motor mount and after 65 or 66 Ford used the four bolt mounts. So your 64 block should have only two bolt hole for the motor mount. Check the date code on the block also. Is this an engine you have or something your looking at? G.