stock 15in w/ 26mm 300zx calipers
 

I know that certain types of 16in 240sx se wheels fit the 26mm 300zx calipers.

But im wondering whether the stock 15in wheels can fit with the 26mm calipers...in other words, will clearance via wheel spacers allow the calipers to fit? If so, how many mm of spacing would i need?

Btw im only concerned about the 2 front wheels

it all depends on the 15 inch wheel you are runnin. if you are runnin on the teardrop s13 wheels, then you need a 5mm spacer. if you have the 7 spoke rims, look on the back of them and check to see if there is one ridge or two on the back of each spoke. if there is only one, you can fit them over the 26mm calipers without a spacer. if there are two, then again a 5mm spacer will be needed.