New Brake Offset

[Rob68427]

Hello
I am building a 68 chevelle. I'm planning on buying new wheels & Brakes. But funds are running low. I currently have 15" ralleys with front disc & rear drums.
Wheel plan is 17X8 - front, 17X11 or 18X10 - rear.
Wilwood Brakes 13" front & 12" rear.
The problem is I can't afford both right now.
If I buy the wheels first, then the brakes.
How much will the new brakes offset the backspacing?



Thanks