this is the info on side of the box my pistons cam in: minimal clearance .0015, skirt size 3.75825 +/- .00025,measuring point 2.61 from top of head. This is all the info that came with the pistons.
OK, the .0015" is the clearance between the piston and cylinder wall reading what I see. Typically the piston manufacturer will have a formula that takes into account the engine/piston use and then provide a "ring gap factor" to multiply your piston diameter size by. Read this article here: Are ring gaps really THAT important?
My generic Pontiac engine book says the ring gap should be between .010" and .020". Your 326 .040" has a bore size of 3.76". Cross referencing this to my 1968 Service manual for the 6 cylinder, its bore is 3.875" - so not too far off. My manual states the top/second ring gap is .015" +/- .010".
The '68 Service Manual says the oil ring gap is larger at .035" +/- .020". My generic Pontiac engine book says the gap should be .015" to .030" - so this falls within the range from the '68 manual for the 6 cyl.
On the oil rings, I don't think it matters which end goes over the other. The oil expander goes between the two flat steel rings (which are both the same) with the flat rings holding it into place. Put the expander ring on first, then 1 flat ring at the lower ring groove locking in the bottom oil expander. Then put the other flat ring at the top of the lower ring groove to lock in the top of the oil expander - think of it like a sandwich.
You want to make sure your rings are orientated correctly as there is an up and down side - some have a dimple, some are by shape. The ring gaps also need to be orientated so that no end gaps line up. The oil scraper will join at the piston pin on the rear side of the piston (piston orientated to face the front of the engine if yours have a specific orientation - an "F" or notch). The the top oil ring will be 1" to the left of the oil scraper joint and the bottom oil ring will be 1" to the right - this staggers all the end gaps so none line up. The top and 2nd ring gap will be orientated opposite/across from the oil ring gaps.
You want to use a ring expander to slide the rings over the piston rather than try and spiral then down over the piston which could damage or break them.
Hope that helps and doesn't confuse you.