There's no such thing as a big block 904. This means one of three things.....
1....He has an RB 426 and is using an adapter of some sort to mount the 904, or what is commonly known as a "super bell." This is where you machine the bell off the trans and adapt it to mount to some sort of aftermarket bell / scattershield
2...He has a SMALL block stroked and bored to about 426
3...He is mistaken about what transmission he has.