Backspacing Offset BS

[Sebambam]

Hey guys
im looking into pro touring style wheels for my
68" Buick GS GM A-Body
fenders are cut and rolled

i found a wheel i really like ( CCW Style)and combo would be
18x8.5 in the front
19x10 in the rear

this wheels are OFFSET measurements
front +20mm
rear + 15mm

so i used a offset/backspacing calculator and converted it to find out the possible spacers i need.

front +20mm= 5.5 BS
rear +15mm= 6.09 BS

Sooo as we know GM A-bodys like 4.5 BS in the front and 5.5-5.75 in the rear, am i right!

in the front i will use 1 inch spacers!
but
would a 6 inch BS in the rear make a big difference? with a 19 inch wheel??
would a 5 inch BS work in the back if i have the necessary clearance??

what you guys think?