I don't think the length of the race changes things. A bad launch in the 8th gen, would affect things. Assuming stock and acceptable launches in both, 8th gen walks away and 6th gen never catches up. 8th gen faster 0-60, 0-100, 1/4 mile, top speed. This is not a debate about which is a better car, simply faster. Just in case anybody is wondering, I like the 6th gen better, but to bet on one aginst an 8th gen in a stock race would be foolish.
I would expect a lighter car with 40hp more to be quicker.
What do the 5.3L cars run the quarter in?
Bone stock the b-bodies were low 15s with a few sneaking into high 14s, a few boltons will net 13s easily. The fastest full 6th gen went 12.3. I doubt there are many fwd cars quicker than that even with engine mods.