Ok mate your '74 if its a TA22 will have a 2T series 1600 motor, and T50 gearbox, and the 81 will be an ra40/45, with an 18R series 2000 motor and W50 gearbox. They will not bolt in and out into eachother.
As for the diffs, they are very closely related as far as i know, being both T292 diffs or very similar, but the mounting points may differ slightly and trackwidth could also be slightly wider.. not to mention youd need to check whether the tailshaft will link up correctly..