Chris
They use the same plugs but need a small tab on the injector relieved to fit.
Impedance was about 4 Ohms on my old set.
Personally, I'd recommend sourcing some new low impedance Sard 650's directly from Japan - by the time you buy some used 550's for $300-400 freighted, clean/flow test them for $100-200, replace the 0-ring/absorber set for another $100, source/make the resistor packs and spend hours butchering the loom you've paid for the new set for $650-700 which are new, come with o-rings and new connectors if required and have better resolution than the 550's in my experience. You'll also have plenty of fuel for 350+rwkw.