Jeff,
That' right, in mid-1958 there was a 430/400hp option, 6v not 8v, 3 2-barrels on top. Very few made, estimates are 100-200 cars total. 1958 only.
Much more common was the 1958 430/360hp 4-barrel, and it was, well, its been called the "Bulldozer" and I agree. Nearly the same for 1959.
My dad also had a 1960 Merc, 430/310hp 2-barrel. I still remember riding in it. BIG. Not real fast though.
Later on we had a 1958 Merc 430/360hp 4-barrel, HUGE difference. If you've been in a 1969-70 Ford 429/360hp Galaxie or Marauder, the early 430 Bulldozer is just about that strong - we had all those cars bone stock back in the day.
I don't know why Merc cut the 430 down to a 2-barrel in 1960. There was kind of an economy binge back then - recession in 1959, little economy cars like Corvair, Valiant, Falcon, Comet coming out, etc, but who wants an "economy" Mercury????
Merc dropped the 430 incher after the Merc bodies got smaller in 1961, and by 1962-63 Merc had the 406 and 427 engines - MUCH better.
My dad built Mercury cars on the assembly line from 1952-1974 so I know them pretty well. He knew them VERY well.