Unfortunately, the location of the spark plug hole and obstructions on the side of the head preclude inserting a rod (even as small as a coat hanger wire) straight in line with the cylinder bore axis. That means the best measurement you could possibly get, by making sure the angle of the rod relative to the bore axis was the same at TDC and BDC, would be:

stroke = rod measurement x cosine A

where A is the angle between the bore axis and the rod.

This will always give a long measurement, which will be longer as the angle between the rod and the bore axis increases. This also assumes a flat-top piston, with no dish or valve relief to change the rod's depth measurement when it touches a different part of the piston head.

If you try to position the rod at a fixed point on the piston top, your measurement will be even less accurate because the rod angle will be different at TDC and BDC (closer to the axis at BDC and farther from the axis at TDC). You are then measuring the difference between two cosines. In that case, the measurement will always be short.

For example, the worst measurement would come from positioning the rod at the far edge of the piston top, against the cylinder wall opposite the spark plug hole, because the angles (relative to bore axis) are the largest. In that case, the measuring rod would only move about 2.25 inches as the piston moved through a 4.0 inch stroke.

I've seen a lot of people get wrong measurements when trying to measure M-block stroke through the spark plug hole. Usually, the measurements are short and they think they don't have a 400.

 

Wow, thank you all so much for your knowledge. since throw measurement is subject to miscalculation does anyone know if the way that oil pan is shaped pinpoints the 351m or 400 issue? I guess the roller rockers can be added at any time (don't know why) along with the manifold and headers. Anybody have a good suggestion on a camshaft that gives me some whoop with the 650 Holley and Edelbrock medium rise manifold???
Bubba I'll have the valve covers off tomorrow, thanks for the hint on head casting though it seems mute at this point. I'll still forward what I see. Thanks again gentlemen, Blessings to you all.