Well, a bone stock 7.3 Ex with nearly identical mileage just sold for $22,500 on BaT - so I'll assume that sets the bar for a stock Ex with ~175k miles. So the big question is how much is the coil spring swap worth to a buyer? I'd say it's worth between $7-11K depending on how thorough the swap was.
For the conversion, were both axles sourced from a newer super duty, or are you running spacers on the rear axle?
Was the brake master cylinder swapped to match the newer axles?
Were the frame reinforcements found on the coil sprung Super Duty trucks welded into the frame?
Were the rear springs upgraded (aftermarket, or '08+ Super Duty springs)?
Were the axles re-geared with running the larger tires in mind?
Depending on the answers to the above, my opinion is that your Ex is worth between $29,500 and $34,000 with where the market is right now.
I've seen 7.3 and 6.0 front coil sprung (4x4) Excursions sell recently for anywhere from $27k (with mileage in the high 100k range) and $36k (with mileage at the low 100k range).
My background on this is that I own a 7.3 Ex that I plan to do a coil spring conversion on.
I also own a '99 115k mile 7.3l F-350 that I've done a front coil spring conversion to.