I have a 1990 turbo II with a street ported motor making 420 hp at rear wheels. I have 6 injectors

550 pri 850 sec and 2 more 1600 that come on at 6 pounds of boost. The two 1600 are controled by a

revbec 4 piggy back. I am putting a ex6 haltech in the car what is the best way to wire the injectors up

Put the 850s in the primary location and the 1600s in the secondary and be done with it. That would be enough fuel for where you're at now. If you want the extra head room you can use the 550 primaries and wire all four secondaries of that pair of channels. One 850 and one 1600 on each secondary channel. At your power level though you really don't need that extra 550cc though.