Will they fit?

Will a 245/35/20 setup work on a stock without raising my car?

That will fit under your GP as long as the rims have the proper offset.... The proper offset is determined by the width of the rim, and I don't know the numbers for oversized rims, so hopefully someone else will chime in.

what offset do i need?

From the FAQ thread

wheel lug spacing and offset

any wheel with 5x114.3 or 5x115 bolt pattern and 38-45 mm offset should work just fine for the 97-03 GPs.

Offset is the flat face where the wheel mounts to the hub and the distance to the inside edge of the rim. The higher the offset the deeper in the wheel will sit, the lower the offset the shallower the wheel will sit.

This link may help so you can PM 2muchspl, he would probably know: Big Rims