I have a set of RPF1's 18x10 +38 all around I have 275/40/18 nittos on the rear rn (came w/purchase of rims) Can I run this offset on my 2008 350z all around? If so what tire size would be ideal for the fronts

 

also car is lowered on BC racing coils

 

You're going to need some fairly large spacers to make a square setup with +38 offset wheels look decent. Most people running a 18x10 square setup have wheels with an offset somewhere between +10 and +15.

 

This. ^^

15mm spacers will work fine (20mm may require some fender guard modification - roll/seal).

Front tire size, 245/40-18 (or 255/40 is fine too)