Intake / exhaust is independent of push / pull (sort of). At 1500 RPM it will not make much difference if you run push or pull, set them up however works best for you.
If you imagine that the case is at the top of the picture, the ones on the left are mounted in exhaust, the ones on the right as intake.
Flushing the radiator is advisable as there is usually some flux left over from the soldering process. Some radiators are a lot worse than others. The easiest way to flush a radiator is:
Fill the rad with a mix of 9 parts hot water and 1 part vinegar (very rough amounts)
Put some stoppers in the ports, pick it up with a towel (it will get warm) and shake.
Empty the liquid into a bowl and see if there is any crud in there.
If there is anything in the liquid, repeat steps 1 - 3
Rinse with distilled water a few times to get rid of vinegar. If you do not have a lot of distilled water then you can rinse with tap water first, then do one rinse with distilled at the end. You will not get any deposits from rinsing with tap water.