Advice is not worth so much when we don't know what kind of truck you're working on.
I've gone through a motor both in place and on a stand. It is really irritating having to work on the motor from underneath, but if the motor isn't too beat up from the start you might get it back on the road a little faster.
In any case, what you will have to do directly is check the cylinders for wear/tapering, to decide if they will need to be rebored. In that case the motor comes out. And gets cleaned and done right. Once you drop the crank, you will want to check the bearing clearances, rods and mains, with plastigage. You can search on these words and read up on the methods.
You really want to take a lot of pictures and keep all your removed parts in very good order. Read the service manual. Search these pages.
Oh, and also, I have always had a machine shop do my valves. Most of these old trucks will have pretty beat up seats and valves.
Good Luck,
Karl
