I *think* you're looking for 0.030" over, not 0.30" (which is more than a quarter inch.)
The formula for calculating the volume of a cylinder is pi*r^2*h (pi times radius squared times height). You're just finding the area of the piston, and multiplying it by height.
And then, since it's a V8, multiple the result by 8.
From: Ford FE engine - Wikipedia, the free encyclopedia
The 390, with 389.6 cu in (6.4 L) true displacement, had a bore of 4.05 inches (102.87 mm) and stroke of 3.78 inches (96.01 mm).
To get the 389.6 number using this formula:
If the bore (diameter) is 4.05", the radius (half the diameter) is 2.025".
The area of the piston is pi*(2.025^2), or 12.88 square inches.
Multiply by the stroke (height) of 3.78", and you get 48.7 ci per cylinder.
Multiply again by the number of cylinders (8), and you get 389.6ci
Boring 0.030 over increases the diameter of each cylinder to 4.08", and the radius to 2.04".
The area of the piston is now 13.07 square inches.
The stroke is still the same (3.78"), so the volume of each cylinder is 49.42 ci (an increase of 0.72 ci)
Multiply by 8 cylinders, for 395.4, an increase of 5.8ci.