If you are changing the pump with the engine installed, DO NOT remove the cover. Instead, carefully remove the pump, and maybe even put long bolts in the front cover that you remove to extract the pump gear screws, to lock the gear in place. It is damn near impossible to line up the timing marks for the injector pump if you remove the gear cover. Take my advice. The cam gear is too far down to see the timing marks. And for the 7.3 to 6.9 pump swap, IT WILL WORK. Obviously, you want to use the right one, but you can use the 7.3 pump. However, you should turn the fuel schedule down which is done by a screw under a cover on the LH side of the pump. Otherwise, you will be burning more fuel than necessary, and fuel is too expensive to waste. You might get more power but unless your pulling a bulldozer, the 6.9 makes enough power as it is.