I am assuming that 57 is same as 56, original spark plugs are AC R45, very hard to find, but you can do a cross reference check at your local auto parts store.
6 cylinder spark plug wires of correct length will be hard to find, when I bought mine ("Xact 2610" at local auto parts store, for 55-62 Chevy 6, same outside as your Canadian Pontiac 261) the black color was fine and lengths were all very good except #5 and #6 were way too long. Sorry I do not have a suggestion there.
Dave
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1956 Pontiac Pathfinder 2dr sedan, 496 - dyno'd 545 hp, stick shift, 4.11 posi - Hot Rod
you could try chevs of the 40's or california pontiac restoration lists 2 sets for your car. c2970721k 1955-58 spark plug wire and boot set,all models,copper core w/modern insulation. c36627r 1955-58 spark plug wires,modern suppressor core wire,all models. both run about 50.00 i think i remember seeing the original cord wires somewhere to,maybe chevs of the 40's or maybe ebay.
I would think a set of spark plug wires would be available through your GM dealership, even though you're in Belgium. As far as spark plugs go, I thougth maybe they'd be AC 44. You can find all sorts of spark plugs on ebay.
1956 Pontiac Pathfinder 2dr sedan, 496 - dyno'd 545 hp, stick shift, 4.11 posi - Hot Rod