The T-slots are to be placed opposite the thrust side of the cylinder walls. If you imagine a piston (front or rear) coming down on its power stroke, the rod is angled rearward slightly in the clockwise rotation of the crankshaft and so the piston is pushing against the rear side (the "thrust side") of the cylinder. So position the T-slots at the front side of each cylinder.

I believe a red dot signifies the piston is for a cast iron cylinder and a white dot is for aluminum. 

Though you didn't ask:

If your rings came with end gap specifications these will likely be far too tight as they were meant for cooler running water cooled engines. Common for newer of these is .003 in. I gapped my rings to .018 in. as the tightest but due to cylinder wear the gaps range .018-.023 through the stroke due to thrust wear. Use a piston to push rings in for gap measurement to be sure they are aligned with the cylinder walls exactly.

Specified clearances that come with modern cam ground aluminum alloy pistons may also be for cooler running engines. George Yarocki recommended generally for the cam ground pistons (I think) .016 - .020 in. clearance (half that on each side) measured at the piston's widest areas which, for the highly variable diameters of cam ground pistons, is usually right under the lowest ring land and from one gudgeon pin hole to the other. A top quality dial micrometer is needed for this precision in measurements. But the clearance amount specs are all over the place for various brand cam ground pistons. 

Confusing this, GY wrote:

"Ten-thousands of an inch clearance was about right for the old, round aluminum pistons. But newer cam-ground pistons only require 4 or 5 thousands of an inch clearance." (The Antique Motorcycle, October 2015, p. 76.).

OK and I've seen .003 in. spec. on these, but that's too tight for these pistons in an hot air cooled engine, right?

BTW I find "T" telescoping bore gauges totally useless for measuring these tolerances. I use a Fowler bore gauge.

If you are not an experienced expert and don't trust what you have done (like me haha) it may be good to have a remote sensing infrared thermometer in use during first break-in to check if anything is too tight. Hottest place to measure I find is at the edge of the head gasket (or top edge of cylinder) - not the plug base, not the exhaust nipple. Maybe begin to worry when reaching 500 deg.F. Worry a lot at 700 deg F. I had a 101 engine feel like it was just starting to seize when this area reached 750 F because I tried to hard to adhere to the tightest ring gaps.

But I'm no expert at all. I only know what I read. And if I didn't try I wouldn't learn. And those are the specifications I used. So I hope to hear from others. 

And I blame all of the above excess on that second cup of coffee this morning.