Hi Nick, in the workshop manual for the FC it lists the Bore at 3 inches.
In the EK workshop manual it lists the bore as 3.062 inches.
So I would think that you are right for a B block that it is standard.
In the parts book it only lists holden part numbers.
Standard FB,EK 7406476
(.003" o/s) FB,EK 7406470
(.010" o/s) FB,EK 7406471
(.020" o/s) FB,EK 7406472
(.030" o/s) FB,EK 7406473
(.040" o/s) FB,EK 7406474
(SEMI-FINISHED) 7406475
Then there are Opt. service 4 ring type with part # M32518
Probably no use to you !
Cheers Stewy