please help with ...20" staggered wheels on my 67 cuda fb

In the front it's going to depend on the backspace. With a 20" rim you won't have to worry about tie rod interference, but the overall height of the wheel could be an issue. Still, I think a 9" rim can be done up front with the right backspace, probably right around the 6" mark, maybe a little more. Probably not much less though.

In the back its going to be tight without a 3" offset and a mini-tub. I was able to fit a 275/40/17 on the back end of my Duster with a 9" rim, that was with a 4.75" backspace and the stock spring location. But it was right on the springs and right on the quarter, and the back of my Duster isn't that low. To tuck the wheels in on the back with a 10" rim I think you'll need at least one of the hanger/shackle offset kits (.5" to .75" depending on the manufacturer), that will probably get you the absolute smallest tire you can put on a 10" rim if you roll the quarter lip. The 9.5" rim would be better, you can probably run that with just the hanger/shackle offset. Otherwise I think you'd be into a 3" offset and a mini-tub.