So what is the difference between a stock 73/74 2.0L muffler and a 75/76 2.0L muffler?

The '73 - '74 Banana muffler has dual inlets for each HE. The flange is somewhat of an oval.

The '75 - '76 muffler is a single inlet from each HE and the flange is triangular. This system is restrictive due to both ports on a side go into one pipe, reverses direction to flow to the back.

Look at the PET, the 75,76 is shown as separate item. 2 into 1 shortie headers then a U bend into muffler or cat converter.

after the stub tubes the gases go forward, then u-turn to the heat exch/heating system. pretty everything is dif except for the copper o-rings and stud nuts. the system weighs a bunch. major pain to remove with lots of rusted fasteners and tight confines.