Hey Paul - I cant say for sure on the Harrison - but different manufacturers may state front space differently - ie. CR states 2.5" front space front and back, but that is to tyre bead - that would account for 0.5" of difference at least.
I measured and test fitted temp and borrowed wheels many times, and bugged just about everyone who had their wheels already, and a lot who didnt, before deciding on an offset I was confident would fit and sit out as far as possible. Ended up going 0.5" larger front space front and back then recommended. So you may find whatever the recommendation may be, there may be room to move one way or another.
One thing I did - after setting the camber "around about right", jacked the car up, took the shock\spring off and lowered and rased the car to check chearance over full suspension travel, and lock to lock on the front.
I have ordered an american racing wheel, I didnt end up going a hallibrand - another style. They make a hallibrand in the offsets you should need, and the price should be similar - worked out at $1750 US including spinners and nuts\valve stems and shipping.
http://www.americanracing.com/wheels...lloy§ion=V
Cheers