Correct weight, yes. It is a match for the engine. If that '93 mustang had a flexplate, that is the right one.
Bolt pattern. It has to be right, even if the Bronco is wrong. You said '77, that should be the large bolt pattern 11-7/16". That is if it is stock. If someone swapped in a '74-76 it could be wrong. I don't have the full history of your Bronco. Even if you have the odd 10.5" bolt circle convertor there isn't a flexplate that will work. The fix is to change to the more common large bolt circle convertor.
The flexplate that works with the AOD that would have been in the '93 mustang happens to have the right balance for the engine, and the same bolt pattern as a stock '77 Bronco. It is also the correct offset (there are 2 offsets because the C6 and E4OD use a different offset than the C4 and AOD). There are also multiple tooth counts available as well. The most common are 164 and 157, but there are others like a 141. The teeth are the same size, so more of them make the overall diameter of the ring gear larger. The starter stays the same. To get the starter to line up it is the bellhousing that locates it. What this means is the tooth count has to match the bellhousing. Good news, the Bronco and Mustang (of the years you are questioning) use the same 164 tooth count.
Pretty much everything revolves around the C4 in the Bronco being the one that a '77 would have been built with. It probably was. It seems like most '77s were automatics. While it was still an option, a lot of people took that option.
