Convert to what? All the L-series heads will just bolt on, given that the external engine architecture is the same from 70-83. That doesn't mean every head will work well on any given block/piston combo, but they'll all bolt right on.
You may need to grind the exhaust manifold holes to match the gasket since the P79 head has round ports and the P90 has square ports.
218,000 miles is just breaking in for a L28 engine.
I have never paid over 150.00 to get a datsun head rebuilt. Valves and seats can be reground, and springs shimmed. And guides, cam, and rockers hardly ever wear out on P79 heads. Call your local machine shop and ask them for the price to clean, resurface, and a valve job. Also get prices for a few valves just in case.
I think you will have more trouble with breaking studs and head bolts when removing a head that has been on for that long.