Agree with the above poster, 4.5L head gasket failure is rare, especially at that mileage. What happened?
I have done the head gaskets on my 89 Deville (stupid radiator drain plug cracked and fell out, dumping my coolant on the freeway. Gaskets were gone by the time the idiot light came on….)
The repair is doable with the engine still in car, albeit space is tight. Getting the rear exhaust manifold off is a pain to wrap a wrench around.
Things to know/my recommendations:
1. There are 2 (3?) higher torque bolts on each head. Many other people have reported that these bolts strip upon reassembly. I chose to purchase the factory helicoil kit and coil the higher torque holes before I put everything back together. You may not want to do that, but understand the risk is there for thread damage. At the least, clean and chase the bolt holes before reassembly.
2. The Fel-Pro kit comes with MLS head gaskets. IMHO, these are fine for machine shop rebuilds. However, if you’re doing a repair with the engine still in car, find composite gaskets. The MLS gaskets have smaller tolerances that require a more perfect working surface usually only achievable if the head AND block are getting machined at the same time. I had the heads machined and simply cleaned the block. My composite gaskets sealed just fine.
3. Have the heads flux checked. The absorb the brunt of any overheating in this engine, and can crack if they do.
There weren’t many people, if any, who had posted about headgasket fixes in 4.5L motors. I looked, because I was most interested in whether or not it could be done with the motor still in the Deville. There were, however, more posts and videos about the repair on 4.9L motors, and is, for most purposes, the same exact repair. Try searching for those and you will find all the really helpful tips and tricks.