Well, from the recent reading I have been doing, 84’s are an odd year being the first year, and only offering the 4 cyl. I think if you are going to do the swap, you should check out Pennock’s Forum (google it) and see. There is a guy there that makes the mounts for the 3.8L that are fairly cheap, but stonger supposedly than the West Coast Fiero kits. And they are relatively inexpensive ($300 or so) with the possibility of doing it for less.
I personnally would start there. Also Series 1 3.8L and series 2 3.8L are different and parts don’t swap. Many people make this mistake and think they do. You want a Series 2 if you want to mod it at all.