It should not go in to 1st gear if you are going too fast. You can shift to 1st gear but the trans will not downshift.
Its also a reason I LIKE using a non band applied valve body. If it does hit first and revs to 7,000+ rpms, you just lift off on the throttle and the motor idles right down. Your brakes should be strong enough to stop safety, if they aren't what is going to happen if you need to stop at 75+mph?
