I'm with Swimdaddy....a bit, heat it, the expansion of aluminium is far greater than stainless steel so they will move apart (in theory sitting here at the PC) but I wouldn't let it cool down, work hot with gloves and a soft faced mallet. Unusual for it to be face to face connection, more usual to be a well machined spigot. Not usual for galvanic corrosion against stainless steel.

If it is old maybe get a nice new one with two speeds or variable and save the electricity cost as well. You don't need big pumps but with no details in your signature cannot see what you have currently.