Consecutive Print to target same output Cell
For[i = 1, i <= 10, i++, WriteString["stdout", i, " "];]
I would manipulate the front end as the code is being run from the front end
For[i = 1, i <= 1000, i++, Print[i, " "];];
SelectionMove[EvaluationCell[], Next, CellGroup];
FrontEndExecute[FrontEndToken["CellMerge"]]
Try the above code. I am at a loss as to what you will do next with the merged cell.