Yup, this is a balancing act.
The most universal, generic, fits-many-engines will be the cheapest to produce (since volume is high, and some machining operations will not be performed) but will require the most professional level of final modification, which places it out of reach of some D-I-Y ("I don't have a good+cheap machinist within 300 miles, it's just me and my drill press"), raises the total cost-to-use, shortens the time frame from order to production but lengthens the time from receipt to installation.
The other side, a dedicated piston with specific valve reliefs (centerline, diameters, separation distance, depth, stem axis inclination) really narrows the number of buyers, but they'll be getting a finished products that can be simply installed (rather than merely Step 1 of a series). The 307 uses 4 reliefs (as do many Pontiacs) because the pin is offset, and this permits a single piston to be used in both banks. Not to insult those who understand this, but the positions of the intake vs. exhaust valve may be left or right so they're made the same (largest, intake) size - which is also true of the 250 (exception: many hemis). Since the pin offset is completely independent of which valve goes where (always on the upstream side of rotation), so the offsets on opposite banks of a V8 face in the same direction.
If it's going to fit the 307 you'll need 4 reliefs, or a centered pin. The most efficient 250 piston (if it has valve reliefs) will be made in handed pairs (left intake, right intake) with the intake relief larger to keep chamber volume low for compression. Note: many engines have extra reliefs for different reasons, the Chrysler "poly" V8 has 2 intake reliefs so a single piston can be used on both banks (no exhaust relief is needed).
