1970 Oldsmobile Cutlass Do I Have Blown head Gaskets?

My 1970 Cutlass Supreme SX, Has a 365 HP, 455 CI Engine, 4 Barrel Carb. W/Dual Air Induction Hood, and Dual Exhaust.
The car from day 1 always had the hot light on, but never overheated, and when I brought this to the attention of my mechanic, he said that it was just an idiot light?
I bought this car 8 years ago, and for the last 3 years it was sitting outside parked in a driveway without being started. Finally, I had some work done in January to get the car going again.
I had all the fluids drained, hoses changed, tuned it up, and it ran like a top.
I did question if any of the gaskets throughout the vehicle would be affected by sitting a long time, and the mechanic said, you would have to wait and see.
4 weeks ago I went to take the car out for a spin, and as I was driving in the garage, I noticed smoke in front of my headlights.
When I checked it out, the radiator was leaking with engine oil mixed in with the anti-freeze. The engine was smoking and overheating, and the hot light was shining brighter than ever.
I only drove the car 4 stories back into my parking slot, and turned the engine off.
I told a mechanic about it, and he said he would do the head gaskets, but at the same time was reluctant, telling me he would charge me $1200.00 to fix, and if it failed, he didn't want to know from nothing since it was an old car engine.
This car has 21000 miles on a rebuilt original matching engine! I want to sell this car for a good price.
How much should it cost to do all the Head gaskets

What can I do? Please help me?
Thank You Tony.