I don't know if I'll be much help with this part, I've not dealt with replacing a dipstick tube in a transmission but I've replaced the oil dipstick tube on my 302. That's pretty easy and very visible so you can see what you're doing. But if I had to guess, you're having a hard time getting it lined up because you can't see what you're doing? The only suggestions I have are an inspection mirror - that might help you see what you're doing from the top - or if you wanted to get really in depth, you could drain the transmission and remove the pan so you can see the hole from the bottom. That would most likely take 2 people for sure.
Now, if you're having problems getting it to start or seat in the hole, you could try giving it some gentle persuasion with a mallet. Be sure to use something like a rubber mallet or, if you have a piece of scrap wood laying around, use the mallet/hammer on that between the tube and the hammer face. You could then lightly tap and see if it starts to seat. I'd advise if you choose to go that route that you make sure it's starting straight so as to not shave pieces of aluminum from the casing. I'd also advise that you pull the pan for that, too, so you can make damned sure that there is no chance of that ending up in your fluid. The hole for the tube is most likely precision fit so the tube won't just fall into place.