i have a 76 400 from my wife's newport sitting on the shelf that had a small diameter, single groove water pump pulley.
i took it off for some reason, but i just looked, and can't find it. i know it's out "there somewhere", because when i took it off, i was after some bracketry for the power steering pump. i then stashed it so "i would know where it was" when i needed it in the future, and of course "now i can't find it".
i'm also thinking there is a small [just a tick over 5" or a tick under 5"] single groove pulley used on some ac small block applications, but i can't confirm. i'm just remembering something from the past, so that may, or may not, be correct.
hopefully, others can provide a correct answer for you.
anyway, consider this a bump.