Change from manual transmission to Automatic

My thought process was to get an automatic transmission from an older international with all the wiring harnesses and control box. I had no intention of buying a new transmission or getting a transmission from a newer model. I have found on the net international’s from the 80’s through the 90’s with the automatic set up that are being parted out. I am not a mechanic and never claimed to be one , but I am more than capable of doing a lot of mechanical things myself. I figured if I bought the full automatic transmission set up off one of these parted out trucks it would not be a big problem to remove the manual tyranny, put in place the automatic tyranny with whatever cross member changes I had to make, install the shifter, tie in the driveshaft, remove the old controller ( computer if you will ) tie in the auto controller etc. either way I need to replace / repair the manual transmission. As far as having an automatic transmission in a truck goes , I drove Fire Trucks , ( Fire Fighting was my living for 41 years until my retirement ) first 15 years the trucks were manuals ( except the newest truck a 1975. When the trucks were replaced they were replaced with automatics. Most fire trucks are not light weight ( medium duty ) they are heavy weight trucks carrying around 3 tons or more of water plus equipment, plus the weight of the truck itself. All that I have driven had good pickup speed, and driving speed. So I see nothing wrong with an automatic transmission for a around town truck. Looked at the replies on here and not doubting any answer , but still not sure if a conversion would be that much of a problem , this truck is just a cab and chassis, with dump body and pto. Nothing fancy , no extras, power steering and heat only