Scufty,
I have a 68' SO block in which I decided to run solid lifters. You are correct that the groove neck does assist in lifter lubrication/circulation. The main idea in plugging the the galleries is to enhance
oil pressure throughout the rest of the
oil circulation system.
Most of the old FE builders will tell you that pressure is wasted upstairs and better utilized on the bottom end. The gallery plugs are usually drilled out .90 if my memory serves. This drilling of the plug keeps enough
oil in the top end while offering enough restriction to increase pressure and deliver more volume to the bottom end.