I think you would be pushing it for an international connection. We transferred UK Int-Domestic at ORD in March this year and cleared the immigration and customs in less that 30 mins but I think we were very lucky. Unfortunately getting into the US nowdays is much of a lottery and the INS seem to know very little about manning the appropriate number of desks for the volume of passengers arriving. You could be very lucky and clear quickly like we did or you could be stuck for ages ( we were nearly 3 hours in Denver last year ) and so much depends upon how many international flights are arriving at the same time. If you only have one possible connection then I would suggest checking to see if there are other alternative routings especially if you are travelling standby. Its also worth taking into account the cost of all the US immigration/departure taxes just to transit the US versus the cost of a ZED ticket via somewhere else.
Just my thoughts ..rgds