Wheel measurements - How much space should I leave for suspension travel?

[BP23G0B]

So I took a few measurements yesterday and It looks like 9.5" rims are not going to fit with my stock spring location (not a big surprise).  However, I did find out the Nitto NT 450 (275/50/17) will fit an 8" wheel.  It's 27.9" tall and has an 11.1" section width.  My currrent rear tires are 255/60/15 (GY Eagle ST).  They are 27.0" tall and have a 10.1" section width.  I'll need to measure again to find out if the 275s will fit, but what should I do about suspension travel?

I took these measurementsfrom the front side of the drivers side drum,
For a 26" tall tire the front side gap is 5.5"
At 28" tall the front side gap is 5.0"
At 29" tall the front side gap is down to 4.25"

So if my gap numbers are correct and the rear suspension were to compress 3", I'd lose 1.25" of tire section width. 

I've got stock springs and KYB shocks.  It's a firm ride with this set up and 3" of travel might be extreme.  How should I account for suspension travel? 

[ShelbyDogg]

I used to have 8.5 inch wheels and would love 9.5s. With the correct backspacing that should fit perfectly.
 One thing you have to watch for is the tire hitting the inside of the INNER Wheel House when going around turns. I had an A-body that I put some deep backspacing wheels on and it would rub the insides with body roll. So if you want to see how much it deflects around turns, put your floor jack under one side's leaf spring and jack up the car with it. That'll let you measure inside clearance. (Be carefull and use a heavy duty jack stand)
Since you said it was already stiff, you shouldn't have to worry about that.