Help altering old Marin MTB to an In-town pavement cruiser?
There really isn't much required to make a MTB into a formidable city bike.

The obvious thing is to change tires. But I wouldn't actually recommend to go for something skinny just something not knobby. Skinny tires offer very little advantage but absorb a lot less of the bumps encountered in city driving which makes them less comfortable and put the wheels under more stress. Just find some nice (semi-)slick tires and you should be good to go (I just posted my preference in another answer and would feel like over-advertising if I mentioned my personally preferred make and model ;-)).

You may think about adding bar ends which offer an extra position for your hands which is quite comfortable on longer rides.

Most other things are personal preference or at least depend on how you're actually going to use the bike. Depending on your speeds you might want to look into larger chain rings for more top end and, since you're a tall guy, possibly longer crank arms.

Other things to think about: lights (hub dynamo vs battery), racks, mud guards, lock.