Based on my personal experience, my guess would be that the head needs to be pulled and the block needs to be cleaned out. There is most likely a build up of material that has accumulated and hardened, severely restricting the flow of coolant through the block. The rear of the block is especially susceptible to this happening. This could be confirmed by using an IR thermometer to check the temperature at the rear of the block as compared to the front.
For sure, it's much better to be able to do this clean out work yourself as paying someone a regular shop rate to do so would probably become fairly expensive since, though it's just mindless "scraping + scrubbing" that's required, it does take many, many hours of labor.
The head should probably come off anyway so that it could be either confirmed that hardened valve seats have already been installed, or to have this done if that's not the case.
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