The 420 cu in (6.9 L) International Diesel was used by Ford from 1983 through 1987, though it did not use the Power Stroke name. Unlike the larger engines, it used indirect injection.
I'm sure the years they are referring to apply to Ford trucks, not buses. I've heard the 6.9's are a little low on power. I have a Detroit Diesel 8.2L Turbo. So far it's been reliable. Fully loaded (hauling two 4x4's), it's able to haul around about 30,000 lbs at 60 mph (although acceleration is quite slow with that weight). At around 18-19,000 lbs (guesstimate) it has pretty good acceleration. With 30,000 lbs I get around 7.5 mpg. It seems I loose about 1 mpg with every 5,000 lbs I add.
Could the 8.2L bus have a 4 speed automatic and not a 5 speed? I'm not sure what transmissions had 5 speeds but it would probably be better than the typical 4 speed Allison (AT545).