Actually, the Cruise doesn't involve downtown Detroit; it starts several miles north of the Detroit border, at Nine Mile Road, in Ferndale, and runs north from there through Pleasant Ridge, Huntington Woods, Royal Oak, Birmingham, Bloomfield Hills, to Pontiac and back - the full loop is about 22 miles round-trip. The heaviest concentration of cars and spectators is from Royal Oak to Birmingham, about 10 miles round-trip.
The most convenient entry point for out-of-towners from the freeway system from downtown would be to go north on I-75 to I-696, then west on I-696 to the Woodward Avenue exit, and head north on Woodward "in the flow". Nice little party - 50,000 hot rods and classics, and 1.5 million of your closest friends

Parking directly on Woodward is hard to find, but off-Woodward parking a block in either direction (east or west) is plentiful. Bring your camera - nobody back home in the U.K. will believe it!