I'm guessing if you need a Currie rear end, you've got enough juice to need some fairly big rubber in there. I don't know what the rule of thumb is for clearance. I should have between 3/4-1" clearance on my inner fender wells with the additional spacers I mentioned. Maybe some of the other folks here could pipe up with their reccomendations.
I have no clearance problems on the outer fenders, but you'll want to watch the travel of the suspension. If your truck is real low, the rear axle may move enough to contact the inner fender above the wheel opening.
Sounds like you've got it figured out - get the wheels, put 'em in the wheel wells and measure.