120? no he didn't do 120. that just isnt really possible, maybe about 80. a .21 will not top out as high as a small block, therefor you will have to gear the car higher if you mean your putting a .21 in a st. also(im guessing you are gona put it in your gt?) your gona have a real hard time mounting a big block in a gt. if you do get it to work you will pry break stuff real fast, like all of the tranny gears, and cvds, plus it shouldnt be a hole lot faster top end, if any.