What your doing is not sensible. The sensible thing would be to get an MTX75 from a wrecker and use it without tearing it apart. Then save your old MTX75 for parts. At some future date if the wrecking yard transmission fails in some different manner, maybe you can take both apart and create a functional one. But you have absolutely no need to do this now.
You have to face the fact, FoMoCus. Your emotionally involved in the SOHC trans. And not in a good way, in a very bad way that is going to ultimately cost you a lot of money. You are at the point now where your not even looking at it logically anymore. "I think the spider gears are the same?" WTF kind of foolish talk is that? You can walk into Autozone tomorrow and ask for them to get both the SOHC and DOHC axles down off the shelf and put them next to each other to see if they are the same, without even taking a wrench to the SOHC trans - you don't HAVE to guess. You can look up the parts now and see they are not the same for the axles. The only reason that you are guessing is because your so bent on making up justifications to yourself out of thin air to justify not taking the SOHC trans back that your aren't even using your head anymore. Your thinking with your emotions - you are absolutely the same right now as those dumb-ass ricers who buy "fart cans" and hang them on the exhaust and convince themselves the "fart cans" are worth 10Hp extra.
Let it go. The SOHC trans is a lost cause. Let it go to some other guy who has a SOHC. Just let it go and walk away. Are you just embarassed to walk back into the wrecker and tell them you didn't know the diffference between the transes? Are you kidding? Do you think they haven't had someone ever do that before?