Since this is your first flathead, a couple things to know (really this applies to any engine):
1) Rod Orientation: -- depending on the engine year, rod type, etc - usually there is a specific way the rods need to go into the engine. The issue is that many have a larger radius/fillet on one side of the rod (big-end) than the other -- this is designed so that the rod doesn't drag on a fillet on the crankshaft journal. So - if there is a fillet, it goes toward the outside of each journal - one rod is oriented one way, the other the other way (always pointing to the outside). Some rods also have oil spray holes - designed to lubricate the cam and/or bore in a specific manner . . . such that they need to be installed in the correct orientation.
2) Piston Orientation: Given that your pistons have a notch in them, as others noted - the notch almost always goes toward the front of the engine. Usually any writing on the piston will be "right side up" - such that you can easily read it when the piston is in the right orientation.
3) Ring Orientation: The rings should be rotated such that the various end-gaps are not aligned (top to bottom). Depending on whether or not your block is relieved (yours is not), then you also don't want the end-gaps to be orientated toward the relief area - as it makes it really easy to break a ring during piston insertion. (It is hard to get rings by the relief area in general - I use a wooden 'paint stick helper' on my race style engines.
Given the above, it is very important that the pistons/rods/rings are correctly assembled - or you can cause some BIG problems . . . ones that can ruin a brand new engine in a very short period of time.
Good Luck - Keep the Questions Coming . . .
B&S
