Often the trottle body bushes are worn, take a spray bottle add some dishwasher liquid, fill with water and with the car idling spray around the manifold, intake throttle body etc, if the iddle changes, you have to look for a leak and possibly get new bushes made.
I would also suggest that you use some carb and brake cleaner spray to clean out any carbon deposited prior to getting the head done.
Check the trottle body is easier to move before and after and spray Q20 into the flaps and bushes.
Once youve reassembled, try at idle again. Give it a rev and remember to check all the body to engine earths, and check the cam timing and crankshaft sensors are propeely routed and plugged in