Don't jump to conclusions..Yes, the factory intake manifold tended to crack under the thermostat housing..The replacement manifolds use a metal cross-over section..Dorman, a reputable old time parts manufacturer makes an after-market replacement that's a little less expensive than the factory part..
But there are other suspects..The heater hoses run in the valley under the intake manifold..They can fail too..
We can only hope the shop that diagnosed your problem ran a pressure test on the cooling system and has isolated the source of the leak..
This engine design places the spark-plugs down inside deep wells in the cylinder heads. The spark plug boots attempt to seal the openings. But if coolant finds its way into the wells, engine misfire is sure to occur and in many cases the coils, there are 8 of them, will be damaged and will need to be replaced at $90 a pop..
Replacing the intake manifold on this engine is a tricky and time-consuming job. Make sure your shop is comfortable doing it and expect to pay around $800-$900...Now would be a good time to replace the spark-plugs too..