I have a '04 1161 with the same rubber latches. One rubber part broke a while ago. Before that, one was always loose the other one took a little extra effort to latch because of the factory placement of the parts. I got a same replacement from McMaster Carr web site, changed the angle on the metal part by bending it, relocated the two mating parts to get a better 'lock' and so far no problems (4 years).
Have you checked you tranny cooler to see if its clean (outside)?
Since you have a Ford you probably already know that there are very few rebuild shops that know how to build a strong 4R100. One was already mentioned in one of the previous post. John Wood in So. California. Second is BTS - Brians Truck Shop in Arkansas. If you really need a tranny, and don't/can't spend the money for one of the above the third choice is a Ford reman. 4R100HD - heavy duty. Period. Visit ford-trucks dot com and look for posts from Mark Kovalsky (ex. Ford tranny engineer) very good advice.
If you gage is reading 60 degrees higher now then it did before I would recommend calling one of the above and chat about possible solutions. They are Very helpful (but also very busy).
The best large Tranny cooler is either one of the large ones from a later 6.0 liter diesel. The deep pan will not do anything for your cooling temps - but it does look pretty. One of the more popular synthetic oils is Mobil 1 ATF.... Change every 30K.