Where did you get that fitting? The block/sender is BSPT and the feed line is NPT. If you try to screw in a standard NPT fitting in the block, it will strip it. You need to get the right fitting from fast-turbo or stealthmode or somewhere if you havent already.

And as someone mentioned above, bigger certainly is not better with IC's. Just like turbochargers, there is a proper size for specific setups.