Lets Throw some info at you first:
Original Truck: 2000 Ford Ranger 3.0l Auto 4x4 (96K Miles Rust Free)
Donor Truck: 1998 Ford Ranger 4.0l Auto 4x4 (113K Miles Front end Damage)
Goal After Swap: 2000 Ford Ranger 4.0l Auto 4x4

So this all started with my buddy hydro locking his ranger. Well time went by and I ended up with it. Now due to a small budget and moderate knowledge about motor swaps I decided to swap out the original 3.0 (Hydro Locked) with a 4.0 out of a 98 ford ranger and the motor did run before I pulled it. I did some research and I knew about swapping the ECM and the entire engine bay harness. Now fast forward a few months. I ended up swapping the Motor, Transmission(auto), ECM, and Engine bay harness out of the donor truck into the 2000 Ranger. I have almost everything put back together but I'm having a few hiccups that I need help figuring out.

1) I've done swaps before and I know your supposed to put just enough back together to get it to start. I am now at that point with all the electrical done and fuel line stuff but when I put a battery in, it doesn't get power anywhere. No lights turn on. The key does nothing. I was wondering if id have to sync the computers together sense the ECM was changed. Any info on fixing this would be greatly appreciated.
2) I need to get the Tranny Cooler lines hooked up but I noticed the old external cooler that is in the 2000 ford ranger has a bunch of tranny fluid mixed with water. I just need to know how to flush the external transmission cooler before I run water in my good transmission I just swapped.
3) I read on a couple posts that I need to make sure I have the right rear axel in it. Info on what I should do there and what the consequences would be if I didn't have the stronger one.

Now please if you have any questions about anything please ask. Also if you guys would be interested in pictures I could see about uploading everything I have on this build.