Although I don't have the trick, I have the Accufab, I would go with the 70 and switch to the 75 later on when your over 350 hp or when you add a supercharger. Thats my 2 and thats what I plan to do with my own. Didn't want to loose the low end trq while I waited.
My personal opinion is no larger than 70mm. If it were me, I'de probably stick with the stock 65mm throttle body until I did internal motor work or supercharged but thats probably just me.