Level_FL350,
I think the assessment does not use much of the 'automatics', so as long as your basic instrument scan is up to speed, you should handle a Boeing as well as you do an Airbus.
The big difference is TRIMMING!
Have your thumb on the stab trim switches and remember to use them whenever you change thrust setting, speed or flaps. Thrust has the biggest effect, due to the under-slung engines.
When you think that you have it in trim, relax your grip and see if the pitch attitude is constant. It's easy to hold a small out of trim condition without realising it until you get distracted by something else and then your altitude changes by 100ft before you know it.
The 757 has a first generation EFIS with an EADI and EHSI but conventional ASI, Altimeter and VSI, so keep your eyes moving! Remember to include EPR or N1 in your scan.
If you find yourself in the 747, it has a PFD and ND quite like the Airbus but you still need to TRIM.
If your brain goes to mush, set 6* nose-up pitch attitude and 60% N1 for level flight. That should work in either the 747 or 757.
USE THE STAB TRIM!