NP203 carriers

[zac]

I got my Mile Marker 501 part-time conversion, be a couple weeks before I have time to install it. The instructions mention this kit is best used with the cast iron carrier as opposed to the stamped steel one. I've read about this before, my question is how would I know which one I have without pulling the tailhousing off the case? The case I'm using supposedly came out of a '79 K20 and it has a th400. My truck is a '79 K20 camper special with full floater. It would make sense if it had the heavy duty part, but I seriously doubt it's that simple. I have my old 203 and I will pull it apart before I tear into the good one. But the problem will be if I don't have the cast iron carrier in either, now my truck is disabled. I could run the steel one, but it would bug me to know it isn't how I want it. Is there some year range the cast iron carrier was used, or is it entirely random? I'm afraid it's like 4 bolt mains, you only know when you see them.

I would also appreciate any tips on other problem areas I might want to inspect when things are opened up. I will do an oil change as well. Thanks.