If you search my username, I have several threads that will help, but the one Ralph linked is most helpful on the biggest issue you face. A 318/360 LA will not clear your radiator because of the long water pump snout. I actually put in a Magnum, but kept the LA water pump because my crate engine was carb'd and I prefer the V-belt look, so from a space requirement, it was the same as putting in an LA block.
Shelby solved the problem with its build by shoving the engine back against the firewall which means new motor mounts and driveshaft, and possibly tunnel work to make the bell of the transmission fit. I solved the issue by swapping a post-91 front end onto my truck. After 91, when Dodge started offering 5.2L options, they redesigned the radiator support and nose of the truck to give the extra room needed. The fenders stay the same, but to do the swap you need from a 91-96, a radiator core, hood, headlights, front bumper and bumper valence. You also need to move the radiator core support holes in the frame forward 3" (this is what gives you the extra room for the LA motor and puts the mounting holes in the right spot for the fenders). https://dodgeforum.com/forum/1st-gen...ert-build.html
The other option is to put a magnum 5.2 motor in there. The magnum with the serpentine belt setup uses a shorter water pump and it will just clear the radiator as is.
As for your other questions:
I have the overdrive version of the 904 (A500) in my setup. I did rebuild it with Ray Asbestos Clutches and Red Kolene Steels and other heavy duty parts to stand up to the 360 Magnum.
For electrical I mapped out everything and completely redid my wiring harness and removed the ECU (I went carb'd). I have a thread on that too and everything that can be removed, and what you need to add to eliminate the ECU. In general, the ECU in my 90 TBI controlled the fuel, spark, AC, transmission, cruise and emissions. I removed the AC to make room for the 360 plus mine is a drop top (don't need no AC). I went Carb'd, so all the fuel management went away...I kept the fuel tank electric pump and used it for the Carb. The Spark is easy...use an HEI GM style distributor. The transmission if you have OD and Lockup needs pressure switches and a vacuum switch to control. TCI makes a kit for that. I eliminated cruise. For Evap, I used a vacuum PCV setup and a vacuum switch to control the purge line to the evap canister as the fuel tank needs venting. https://dodgeforum.com/forum/1st-gen...onversion.html
Bottom line, it was a fun project for me, but if you're looking for a quick inexpensive swap, it's not going to be one. You're better off finding a junker 3.9L and rebuilding the engine.