1962 40 HP Johnson outboard driveshaft seems to be stuck to power head. How do I get them apart to replace Impeller and seals?

Have you taken the inspection cover off the housing and disconnected the gear shift shaft so that it just slides out  !!!!!!!!!!     The plate is about half way between the cavitation plate and the underside of the engine block. The plate is about 2 inches long and held in place by 2 screws. Once removed you will see a brass coupler with two screws . Undo the bottom screw and then pop it apart. A warning put the shift in reverse and when you reassemble make sure you have bottom end in reverse when you go to reassemble. Don't cheap out and just replace the impeller, buy the whole water pump kit and it will last you a long time.

Soak in PB-Blaster for 2 days.

Firmly secure the power head to a bench or a solid trestle and then use a wooden mallet to hit the lower unit firmly in different directions. 

If that fails try a bit of heat: shield either the lower unit or the power head using wet towels and use a hair dryer on hot to heat up the uncovered metal near to the joint. A conventional hot air gun will also work at the risk of paint damage. 

Once the head or the lower unit have been warmed up try a few sharp taps again with a wooden mallet or use a conventional hammer with a wooden block. NEVER use a metal hammer to metal castings directly. 

Penetrating oil like Plus Gas can also help, or use diesel/paraffin. Don’t waste WD-40 on it as despite what the can claims it’s a poor penetrating oil and a rubbish lubricant. The primary purpose of that stuff is as a Water Dispersant (hence WD).