I'll attempt to answer.

A lot of people were using Chevy 400 cast iron small blocks.

The 400 Chevy had a bore size as 4 1/8" and a crank stroke size of 3 3/4"

The largest you could safely bore the stock Chevy block was .060 oversize making 408 cubic inches.

To include the growing popularity of Aluminum blocks and to keep the iron blocks competitive they settled on 410 cubic inches. Most teams using an aluminum block then were staying with the Chevy bore size of 4 1/8" and increasing the stroke size to 4 7/8" to make it exactly 410 cubic inches

Not a good explanation I know but on just my first energy drink this morning.


.