Here's a thread with step by step pictures and over 1,500 posts:
https://www.mazda3forums.com/61-powe...highlight=swap

 

Read what he said in post #7 bro. He needed to use half the mazda motor. I completely agree its a dope swap to consider but u can't get away with selling your 2.3 and throwing in the 2.5. Especially if ur mazda 3 is before an 06. I'm certainly keep this swap in mind if I start having issues with my 2.3 later on but this isn't a cheaper or better alternative to just replacing my transmission. I got one for 200$ after tax from the wreckers. The 2.5l engine alone wud cost more than that plus I still need a tranny lol. If you have any tips or tricks concerning the transmission replacement that would be really helpful tho.

 

You are reading from very early in the process (like years ago) of figuring out how to do the swap. Skip to the end, it is a very different story.

 

Tranny swap is just like changing the clutch. Just follow the clutch changing instructions and you can swap the tranny. Definitely have manual for the steps. Here are my tips and tricks for the tranny swap (assuming you are doing it in your backyard garge and not at a professional):

1. Jack the car high enough so you can remove the tranny from underneath. You will need to support the engine too when it comes out. Use jackstands, don't DIE!
2. Remove as much as you can from the drivers side, battery intake etc. I found I could work around the electronics without removing everything the manual recommends.
3. Be sure to plug the slave cylinder, so it does not leak all over.
4. Be careful with ball joints.
5. Replace the clutch, TO, and pressure plate, don't be cheap now or you will be doing this again.
6. You will need an impact or some serious breaker bar for some of the bolts.
7. Have a helper for installation.
8. (optional) I painted my tranny before install and it looks really nice. At least clean it up.

Personally, I find doing the clutch swap is easy and you can fix things as you go. By myself I did it last time in about 8 hours with lots of breaks and chatting with friends. Have extra time in case something goes wrong, nothing sucks more than having to hurry in the dark.

Good luck,
RGAZ