I have OD guts in an iron case in my 68 Charger/440 engine. Did the swap over 10 years ago but not tons of miles on it. (with 4.10 gears the miles are more pleasant) At the time I rebuilt the trans the reading I'd done said the aluminum case allowed the countershaft to move away from the mainshaft and that's what did the damage. The article I read talked about machining the aluminum case to spread the load. Its been a really long time and I cant remember the details. In an iron case with street tires and reasonable driving I wouldn't worry about it.