magnum 360 into 79 powerwagon ?

Hi guys:
Iv'e a 79 plumb crazy stepside powerwagon,with over 200k on it. It is sucking an exhaust valve for the second time. When I fixed the last valve I noticed polished cylinder walls and heavy ring groove. I found a 93
360 magnum with 72k on it. I can buy it right with an OD automatic attached. Am I asking for trouble , or will this bolt in relatively easily? I have a magnum four barrel intake already and would like to use the new headers that are on the 79 360 engine. Is it feasable to change out the mainshaft in the trans to put the OD in my four wheel drive powerwagon? I know enough to be dangerous , but I also know enough to ask people who do know. I just rebuilt the 727 and it works great. I was thinking of gas mileage. The powerwagon is full time 4x4 with the 360 and averages 10 or less miles per gallon. I'm not afraid to work, just don't want to waist time if it isn't a sound Idea. Shouldn't the magnum do better on gas, with the better heads and performance ? Iv'e written a book and it's late, any input as allways would be most appreciated. Thanks
Ken:)
Author: ken