Another option would be to park at the Park and Ride at East Didsbury Metrolink station in Parrswood if you’re coming from the South. The car-park is massive and of the two lines there, you can get either tram on whichever line.
For the Science museum you could get off at Deansgate-Castlefield stop, its probably about 25mins walk. Journey from East Didsbury to Deansgate-Castlefield about 25mins and tbf its actually quite a decent view going over Castlefield and then past the new hi-rise towers in Deansgate Square.
If I’m driving I park at Dale Street near Piccadilly which would be close enough to the football museum but a decent walk to the science museum (though easily done).
Whatever you do, don’t park at the Arndale Centre multi story. If there’s another big event on like football, markets, gigs, whatever, it is possible to get trapped in there for hours while the traffic logjams outside.