porsche wheels on bus

hello,

i have a 75 westy bus, and i want put porsche boxter wheels...

i have them, and 2 wheels with tyres (205/50/17)... so i've test them on my bus....
on the front, no problems...

on the rear, no problem too, but it pass just, so just my finger pass but without my ring (if i keep, it doesn't pass ring diameter of 2cm)... i would like buy 215/50/17 tyres, but i not sure, it will works...

my wheels are porsche 7x17 ET55, adaptor vw/porsche of 23mm

what do you think..??? does anybody have experience in it..???

thanks

fred