It is a slight stretch but nothing too extreme (I had 235s stretched on a 10 inch rim on my last car
)
I had a similar experience to you actually when I wanted to put my 215/245 tyres on the 8.5/9.5 rims. The shop I went to refused to put them on because they said it would not physically be possible to fit the tyres on the rims, I went to the shop next door and got it done in 10 mins
I'm not sure if its a good idea to run 225 on one side and 215 on the other though! its probably best you do both of the fronts at the same time if you're planning to go up to a 225 (i.e. dont have the front left and front right being different sizes)