Thanks for the replies.

I don't know exactly how the hp usage is worked out - it can't be a constant, so maybe the highest loss isn't until full-on at 5-6000 rpm?
The more I dig, the more the 60ish hp figure comes up. And around 15hp for the 4 speed transmissions.
And yet one guy with 302 in an older F1, swapped from a 4spd to a C6, & said he couldn't notice any power loss driving..........he wasn't sure on his mileage.