I've got Michelin Krylions in 700x25 on my '87 Ironman Expert. They measure out at almost 27mm wide on the stock rims. They only have a few hundred miles on them, and there's about 7mm of room between the tire and the fork - brake looks about the same clearance - and about 5mm between the rear tire and the rear brake. I think you could fit a 28 in there, but Lord help you if your wheel goes out of true or you break a spoke!
IMO, you'd be better off with a different frameset that has the room to comfortably run 28s than to try to shoehorn them onto the Ironman. Either that, or run 25s...I've done Rouge-Roubaix twice (not on the Ironman!), once with 25s and once with 28s, and didn't notice much difference. That's 105 miles with 20+ miles of bad dirt/gravel roads. A tough 700x25 (I used Gatorskins) will do fine.
__________________
Regards,
Chuck
Demain, on roule!