That's a good question. I've never seen inside a 450MHz iMac.
Perhaps my "guide" is a little generic... for all I know it could be all 400 & 500MHz models made after a certain date, rather than determined by clock speeds. I know for a fact though my 500MHz Indigo logic board fit into my 600MHz Graphite body shell, but my 400MHz Ruby board didn't. However the Ruby board swapped just fine with the 350MHz G3.
The only issue is in the position of the CPU heatsink. Since the heatsink is physically part of the dividing plate that holds the PAV board, it can't be easily relocated without transplanting the entire divider board (in which case it makes more sense to swap out the PAV only instead).