One of the settings on the last page of the shipping behavior is Quantity, this is the quanity of boxes that will ship.
You could use a calculation to dynamically break the shipment in 70 pound intervals, for example:
<?php echo ceil($yourCart->TotalColumn("TotalWeight") / 70); ?>
would set the quantity to 1 if the total weight is under 70, if it is over 70 lbs, say 90 lbs, it will set the quantity to 2
then for the weight, you could use this calculation:
<?php echo $yourCart->TotalColumn("TotalWeight") / (ceil($yourCart->TotalColumn("TotalWeight") / 70)); ?>
which would set the weight for each box dynamically, your 90lb cart would now ship in 2 boxes, each would weigh 45 lbs