wowser

I amgetting the ford racing 5 lug conversion kit for my 89 mustang installed on monday.i need to order wheels and tire today so i can have them here for next week so can go to the local drags on Saturday. Here's the question, the wheel guy says the stock offset is negative 25 he can only get wheels with a 40 negative offset. Does the stock offset change with that kit and if it does, what does it change to. and is the offset number in milimeters. any help out there would be appreciated