How much will a 27×10.5 slick grow?

i can tell you that a M/T tire will fit in 10" width. 10.5 will be very tight but might work as all cars are a bit different.
A 10" wide Hoosier won't fit. Not at all. Need a spacer under the rim and a lot of bending the wheel housing outwards. I made that experience last week and have Pictures.

My car is a 72 Dart. Springs etc all stock