460s up until 1979 were internaly balanced. I had to use a 390 or centerforce flywheel with my 1971 460. I chose the centerforce (40.5 lbs).
When I junked my knocking 400 in my bronco, I gained about 1.5mpg with the 460. Although 10 is nothing to brag about.
You have to watch out on some 400s from the junkyard. The 1972 400 (or what ever the first year production was) had a small block bell housing.
To answer the question BB or SB, it's neither. It's a 335 series Ford engine. The 370/429/460 was the 385 series. The 330/352/360/390/406/410/428 was the FE series (Ford Edsel). Confused yet?