They are definitely RH thread. The tensions are not particularly high, and there is no good reason they would be loktited. Is your problem with the 1/2" bolts or the 5/16" ones?
Best guess if it is the ones that go into the block is that they have got some water on the thread, possibly due to a coolant leak in the head gasket, and are rusted in place. You could try moderate heat on the bolt heads. You are unlikely to brake the head bolts, but the smaller ones may break fairly easily.
John
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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