My tank is similar in size (70x20x24) and has no center cross brace. Euro bracing is ~2.5" wide using 1/2" glass.
Your flow will probably be under 1000gph so really don't need a drain bigger than 1.5", also could consider 2x1" bulkheads for return and Herbie backup. This would allow you to keep your Overflow chamber fairly small.
Check RC Headloss and Drain calculators, they will give you a recommended linear weir length to use for planning the chambers dimensions.
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