Back space question 67 Chevelle

[6t7vert]

Hello, after seeing 907rs 's wheels that made my selection clear on which wheel to purchase!!

I also selected and put a deposit down on Budnik Fontana's. I have chosen 18" wheels.
The manufactuer suggested based on past installs:
F- 18x8 4" B.S. 235/40
R- 18x9 5.25" B.S. 275/40
I plan to lower the vehicle after the install.

Any thoughts or opinions on tire size or
BACK SPACING????
I only have a few days to make changes if needed!!!

Thanks for any help or opinions!!
Kevin