You can go as big as you want but you wont feel a thing from going 65 to 70 or 75mm, dosent the crate motor come with a cobra intake? The cobra intake is just bearly over 65mm opening, sticking a 70mm t/b on there would be fine but a 75mm would be pointless, you wont lose or gain anything, id stick with the 65mm, its all you need and its the same size as the intake opening, and its new.
EDIT: sorry i thought you said the crate motor came with a t/b. Are you going to be using your eddlebrock intake on it? Realistically youd like to get a t/b the same size as your intake opening what ever intake your using, but just use the 70mm imo.
Short answer is NO, the 70mm will work just as good.