You dont have to tear the motor apart, just pull 2 spark plugs.

Put #1 cylinder on TDC, pull the spark plug and stick a long pencil or other such skinny rod down, mark it where it hits the flat by the hole. Put the rod in the #4 cylinder the same way, without turning the crank, and then measure the difference.

3 3/4" (3.78 stroke) is a 390, 4" (3.98) is a 410 (428 too).

Just out of curiosity, I've never seen a C6AE-L head, or even heard mention of one. I have a set of the C6AE-J's, but I would like pic's of the -L's, if there is any way you could take some it would be greatly appreciated!