will a 250 rear tire fit on a 2016 pro street, i have a half in. clearance between my belt and tire 
It depends on the tire; each brand has different dimensions. Find the brands you want to consider and get the specs for the tires, then compare to the stock tire and the measured clearances you have with the stock tire to the belt and any other parts.
The stock Dunlop D407 is 240/40R18 79V, the width is 9.35", and the overall diameter is 25.86". I looked at the Dunlop Elite 3 250/40R18 in the catalog and they list the width of that tire as 10.02" and the overall diameter as 26.20". They recommend an 8.50" rim width for that tire btw, and the stock rim is 8.0".
Without accounting for any variation from manufacturing, that means the 250 is 0.67" wider, or 0.335" per side. You indicated you have 0.50" clearance with the stock tire to the belt. Subtract the 0.335" difference from your 0.5" and you get the theoretical clearance to the belt with the 250 Dunlop, which is 0.165" or approximately 5/32 of an inch. Personally, I wouldn't feel terribly comfortable running less than a quarter inch clearance, but it's not my bike and not my decision to make.
Jerry