mespock - Here, simple: Keep the 85 pretty and confortable. Use it as a highway cruiser. Get a 1988+ if you want a potential fast car that's still a Lincoln. The only way power is getting out of that old ass car is via an engine swap.
The Thunderbird should have come with a standard 5.0L, not a 5.0LHO. If it says HO on the intake plate, you either got a swapped engine or intake.
If you really want a fast car with real potential and ease of upgrade, forget Lincolns all together and get a Stang. I love my Lincoln, don't get me wrong, but it really is more difficult to get fast. If you like the challenge like I do, then by all means, continue. But don't try to upgrade that dog of an engine. The heads are poo, the cam is poo, and the fuel/air system is poo.