All credit goes to John Donne, whose poem I'll never forget!

GMAT be not proud, though some have called thee
Mighty and dreadfull, for, thou art not so,
For, those, whom thou think'st, thou dost overthrow,
Die not, poore GMAT, nor yet canst thou kill me.
From rest and sleepe, which but thy pictures bee,
Much pleasure, then from thee, much more must flow,
And soonest our best men with thee doe goe,
Rest of their bones, and soules deliverie.
Thou art slave to Fate, Chance, kings, and desperate men,
And dost with poyson, warre, and sicknesse dwell,
And poppie, or charmes can make us sleepe as well,
And better then thy stroake; why swell'st thou then;
One short sleepe past, wee wake eternally,
And GMAT shall be no more; GMAT, thou shalt die.

I think you'll find that you simply don't have enough time to dream and worry during the test. Even if you have jitters when you first sit down, once the essays come up on screen, I think you'll find yourself calm down. Most importantly, though, just take it one step at a time. Break those daunting 4 hours into more manageable chunks of time: at first, just focus on planning strong essays. By the time you get to quant, you'll have found your rhythm.
It sounds like you've prepared yourself really well and worked really hard. Just mske sure you go to bed early the night before and bring food and water with you to the Pearson center. Kick GMAT butt!

Most standardized tests are curved for people with confidence (false or verifiable). So, here is some advice that might just give you some confidence:

1 - Although 750 is 98th percentile, you aren't competing with the other 90% of test takers. Yes, 300k people take the GMAT every year, but the majority of them just need a score between 550 and 650. They don't even have hopes of bathroom duty at Stanford.

2 - You are obviously smart and have done reasonably well on your practice exams in order to have expectations of such high score. Rely on the instincts you've developed, your wit, and you'll be fine.

Best of Luck!