Been there done that..............except I converted an 85 NA Regal backwards to an 82 TRSC.
Here's what happened to me:
Installed 82 rear end and TH350 transmission. Stripped the NA motor down to its long block then installed the 82's intake, turbo setup, down pipe, etc. I had to drill and tap a hole on top of the block (in the rear) to install the 82s knock sensor. I swapped the 85's computer and harness over to the 82. The main body harness inside the car plugged right into the computer. Everything worked fine until I wanted the 82 dash to complete the clone. The main body harnesses connector that plugs into the back of the dash is wired differently between the 82/85. I thought I could just rotate it 90 degrees (which I did and it plugged in). That stunt made the 82 temp gauge flicker in sync with the blinkers, the green bliker indicators would not work.......I forget what else was odd. Upon further inspection I found the problem, the dash was not compatable with the 85's main harness. Wanting that dash caused me to replace the main harness inside the car and the harness outside running to the lights, ign etc. due to the incompatability of the firewall connections. Since I swapped the engine side of the harness I had to install the 82 wiper motor as the connectors were different. Two of the 3 mounting holes lined up prefectly, but had to drill the third (easy). At this point everything worked perfect except no LOW speed on the wipers. I ended up installing the 82 ignition switch on the column to fix that. Even though it plugged in, the 85 ign switch was wired differently. Also the ESC module on the 82 is somewhere under the dash between the steering wheel and the radio (or was it behind the glove compartment). Before it's too late, use wire markers to label/record everything you disconnect or to identify things that were not used.
Depending on what your intentions are.........I recommend buying the whole car. Slowly take your car apart and swap in the 79's stuff. I ended up using a lot of parts I had not intended to, luckily I had stripped it clean before I sent it to the boneyard. Buick likes to swap connector styles, pin sizes and styles between years making it difficult to plug-n-play. Worst of all, they lalso like to use the exact same connector only wire it differently. Pay close attention and you'll see what I mean.
I thought my conversion was relatively easy overall because it was 99% plug-n-play as I had all the parts. It did take a little while to work the issues noted above.
Hope this helps.
!
