If its a 205 block with small core plugs then it will be a rs500 block just probably not from original rs500 road car if no number. Would have thought many were bought from ford before 200 block was available, if the head is genuine it will be dowelled to the inlet manifold, although I don't think ports or valves are any different to std 2wd head. Good basis for a high powered engine though if all ok.