[84LX89GT]

I believe that exhaust wise for what you're looking for is that the '85's had dual, single, dual exhaust (went to Y-pipe under crossmember and back out to two outlets) whereas '86 was the first year of true dual exhaust. You may have to change the transmission crossmember if it doesn't have 2 "humps" for pipe clearance. an easy way to tell this is to look at it and if one side looks similar to the other, then it's dual hump, if the right side (i believe) is flat, then it's a single hump crossmember. You may also have to get the matching hanger for the H-pipe. They made a different hangers that bolted in between the crossmember and trans that are specific to either a manual or automatic trans. The stock one you might be able to use, but it will only support the left side exhaust pipe if it is for single systems.
Ford racing sells a dual hump crossmember kit, also it's pretty cheap to find at junkyards if you don't want to pay for a new one. Also the exhaust hanger is easy to find as well. When i converted my '84 LX i bought it all for something like $35 dollars.
Good luck, hope this helps.