If you do a leakdown test before tear-down, that can tell you where it's leaking, also condition of rings and valves, but you're going to find that out anyway on the tear down. Leakdown test rather difficult on SC with the tight fit, all the tube routing. I recently bought a '91 SC with the same problem for $300. Gaskets(top end set) and the stud kit>(highly reccomended) from Bill @ supercoupe performance set me back close to 300, other miscl. brought parts to around 4.You're also going to need the teflon intercooler tubes tape Labor was 450, which I considered fair. If possible, whether you or a shop does the work, have a factory service manual, it really helps. So after repair, car runs great but still gets a bit hot. Found very small antifreeze leak at thermostat housing. A lot of times, something else in the cooling system, radiator, water pump, etc. can cause overheating, which can lead to blown headgaskets. A lot more common on 89-93 SC's though.It's pretty rare for cracked heads or block to be the cause. When something breaks, I like to find what made it break in the first place. Usually a little problem , not taken care of, leads to a bigger problem. Gotta fix them all. Just tear her down, see what you got, and order the parts. While its apart, its a great time to check the cylinders for wear, ridges or scoring, and get the valves done if it needs it. Take everything off as an assembly wherever possible, charger, TB, etc.
Mine had blown gaskets on both sides, pass. rear cyl. and driver side middle.
Still have to replace the O2 sensors, to pass smog, too.
Best of Luck with repairing your SC, hope this helps......