yes the copper washers will be your issue make sure you clean all the carbon off and out of where the washer seats in the head refit with new hold down clamps and bolts if you dont know if the modified ones were previously fitted
are you 100% sure you have got the injectors seated fully did you use the correct copper seals
we have never done rings on thease engines and i have customers who have done 350,000 miles plus, we do do lots of injector reseals and i know how difficult is it to remove the coke build up from the bottom of the injector port we often don't get them to fully seal first time if they have been leaking bad also had a customer who used incorrect injector wahers and that caused a problem with not enough clamping pressure on the injector
does the engine smoke when driven? if its a piston issue then it will
have you done a compression test?
i would still put my money on the injectors not seating