Are you sure on the factory RT40 offset? Seems odd to be that negative. I know one of my cars has a zero offset and is very unusual for a factory car (fun trying to find wheels). If that 30 neg is meant to be 30 pos then you would have a diff width of 1330 and would fit with your 50 pos wheels on a 1370 diff. Biggest thing to consider is how much space you have for the inner and outer guards, width is just as important as offset. Remember if you compare a 7 inch 50+ offset wheel to a 9 inch 50+ offset wheel, the track will be the same but you have an inch extra of rim each way. If you are limited in outward movement but have space inwards you would need to run a 75+ 9inch rim to keep the outer edge in the same place.