You didn't state your tank dimensions, but I think I can answer your question.
If the tank is 1/2" glass and only 20" high, you don't need eurobracing for a 75g. Take it off and leave it off. This is somewhat dependant on the tank dimensions, so better post these and i'll double check.
I'd be really hesitant to put a 4" euro on a tank that size. You will really limit your access.
If you really want the brace to hold your return plumbing, put a triangle piece in the corners.
The tank I just had built is a little bigger, but I learned a lot in the process. If you interested, you can read about it here:
http://www.canreef.com/vbulletin/sho...66&postcount=8 - There are some notes on glass thickness in the post.