It uses special (I think from a Ferrari) plugs with a 10mm thread diameter and a 13mm socket size. I don't remember the number. But a socket won't come in, at least with our homemade FI manifold. We had to use an open end wrench to start them and turn them with two finger tips. They sit at a slight angle under and just to the side from the inlet manifold, and of course just above the head gasket. Of course the generator, distributor, starter and dipstick are all in the way.
Every time I think about doing individual inlet ports, I remember trying to work on that thing. I think I might just put steel plate spacers in the standard head to separate the siamese inlet ports when I get around to doing efi.