Rim and tire size ?

[Where's my damn beer]

Hey everybody, I'm currently building a longbed 1970 cst10. It is lowered 4 1/2" in the front and 5 1/2" in the rear.Its about time to order rims and tires and I was just curious if anyone knew what i could fit under there. I dont mind tubbing the back if necessary, just want to make sure I clear the frame and the outer wheel well. I was looking at 20×12 in the rear and 20x10 up front. Not sure about offset...

Thanks