I just replaced the manifold on my H this past weekend. All went well. I didn't need an impact wrench or anything. I used a Black & Decker polisher called a "mouse" to clean up the surface after removal. Put on the new gaskets and made one to fit the carb connect. Bolted her on and torqued the nuts to 50 lbs. Now she purrs like a kitten. The manifold is now the cleanest and best looking part on the whole tractor. :-)
Have fun and good luck!