Congrats man!
I would not use the 20mm on the rears. They will be seriously pushed out. 15mm would be the max and that is pushing it. See
Minicobra1's gallery, he has 15mm on his Nismos. Personally I would go 10mm front and rear IF you really want, but IMHO spacers are not needed with these wheels.
I would get 305-30's for the rear for sure! Hankook V12's are popular...