Good question. since I am not familiar with your model year, I would recomend that you contact an expert. Try this link
http://www.sacramento-mustang.com/
Call them and ask for Shawn or send him an email. I think you will find the answer. I can say that with the 2000 GT, everyone had been only recomending the 70 mm TB until this new plenum became available and they are now saying to go for the 75 mm. I think you will see a lot of 70 mm TB on the used market for sale in the next few months.
As far as the 80 mm TB is concerned or larger, I just don't know. Let me know what you find out.