var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Sheet1");

var data = "some really long string greater than 50k characters";
// write content to a new row created, no 50k error!
sheet.appendRow([data]);

var lastRowRange = sheet.getRange(sheet.getLastRow(), 1);

// copy newly created content to desired range
var outputRange = sheet.getRange("A1");
lastRowRange.copyTo(outputRange);

// clear newly created row
lastRowRange.clearContent();