I would swap in a 617 which can be had for a few hundred. Replace all gaskets down to long block while you have it out of the car. Of course, don't mess with the head gasket. Do adjust valves and check timing chain stretch before install. Refrain from making a lot of changes to a good running engine. Only stop the leaks.

Alternatively, send your injectors off to be pop tested or make your own tester. Professor Google has lots of information. You can loosen the hard line at the injector while engine is running and see if the nailing becomes obvious.

Install all of the injectors at once after you know that they are good. There is no point in removing the valve cover 4 times to do an injector install. (Edit: I was thinking of the ram where you injectors are under the valve cover.)

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.