Stock bore (diameter of cylinder) is 4.00" for 289/302/351. When rebuilding, the diameter of the cylinder is increased to make sure it is straight, because years of use can warp a block. .030 over means the bore was increased to 4.030" It is like re-drilling a hole larger to make sure it is the right shape.
Boring requires pistons for the new bore size.
Boring also increases displacement. Your 351W will now be 363 cubic inches.
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67 Fastback - Arctic White Pearl paint
351W ,Trick Flow Aluminum Heads, Edelbrock TorkerII, Carter 750 CFM, Comp. Cam 477/510 219/233@.050, Performance Automatic C-4 Trans, 3.55 gears, Front Disc Brakes, 1-1/8" Fr. 3/4" rear sway bars.
My 351W Fastback