Cutting hole in hood?

Not sure what kind of hole you plan to cut, but here's pics and description of what I did for a blower:

http://www.70duster440.com/hood.html

It's all tied to the inner structure and isn't flimsy at all. I did retain the 'light' hinge springs I used with a glass hood before and they hold it up. Engine flex will be determined by the motor mounts you're using. You shouldn't need a ton of clearance - even with old rubber biscuit mounts.