What is the best way to mount an electric fan(or two) to a 75 vette?? or is it just better to get a Flexfan and leave it as a mechanical?? Is it worth the 17(or so) Hp??

Mounting depends on the type fan you us. Either a push or pull type. That is usually dictated by the amount of room you have in front or behind the radiator. Pick a couple of manufacturers and call them. I do not know how much HP you might save, but I am sure you save some. Claims tend to run to the high side.