The bumper height varies even from model to model on trucks leaving the factory, so I don't think altered vehicle/bumper height is regulated. As long as they meet the no-damage at (5kph maybe??) rule, then you're good to go. Call the Ministry of Transportation for info.

A couple heavily modified trucks pulled in to the local car show the other day with bumper heights pushing 6'

Don't know of any bumper height restrictions in Ontario (they exist in BC and NS, probably more provinces), but your front headlamps must be between 559-1372 mm above the road surface and your rear taillights must be between 380-1830 mm above the road surface, measured from the centre of the housing at vehicle curb mass. This is specified in the Motor Vehicle Safety Act and applies across Canada.

There very well could be bumper height regulations in that same act, but it's a confusing document that frequently references the FMVSS, the American counterpart.

I know this is a 10 year old thread, but, the content is still current.

You will need to check the federal safety laws for the laws regarding headlight height and bumper height. Back in the early 80s in Ontario there were changes to the HTA that dealt with vehicle height. I am going from memory here, but, there was something like vehicle height had to be withing 6 inches above or below stock height. There are specifications available from the manufacturer that will define "stock" height. This was brought out to combat the raised 4x4 trucks that were all the craze back then. The vehicles that were obviously modified, like the vehicle in this thread, were easy targets.