change manual to automatic.
but some of the flyweight 's weight is compensated by the torque converter, is it not?
Yup!
It's pretty much a straightforward swap.  Depending on the year of the two trucks (the key is to try not to bridge between 1995 and 1996 'cause that's the big change year.), The ECMs should be compatible although I'm no expert at that.
You just bolt the flexplate onto the end of the crank then slip the engine in place.
I did all the engine removal/installation with the transmission in the vehicle.  It might be easier (in one way...) to put the engine and transmission together outside of the truck.  There are several braces that bolt between the engine and transmission (at least for the 4-speed I worked with and I suspect they are there on a 3-speed, too) and it might be easier to deal with them that way.
You'll also need the radiator with the transmission cooling core and associated plumbing.
Remember to take a good look at how everything is connected together on the donor truck.  Take pics and lots of notes!
If you can, get your hands on a GM or Suzuki FSM for the donor truck.  Lots of useful info in there!  Especially about the two types of automatics.  I had access to a  '96 GM FSM that proved very helpful!
Finally, replace the input seal on the automatic as they tend to get buggered up.  12-15 bucks is a good investment!
Good luck!