Thinking about LS3 Swap
 

I'm considering buying a 2003 Miata and swap out the engine with a Chevy LS3. Outside of the engine, are there any swap kits I need to purchase? I've heard of people doing it without. If I wanted to keep it manual and not automatic, what's a compatible transmission? I'm new to this sorry about the many questions.

WELCOME!! Do yourself a favor and read through the thousands of posts on this site to find the answers to all of your questions. There are many nice build threads on here as well as some videos. Whatever money that you have budgeted for the swap, expect to spend twice as much.

X2 what Tom said....

unless youre dead dead set on doing the swap yourself, I have found that sometimes it can be more cost effective to buy a done car........an option worth considering...

Most of your questions will be answered, plus a bunch more you have not thought of yet, if you will go to YouTube and watch the video series by Turbo Tom where he does the very swap you are interested in.
It is very informative and will allow you to decide if you have the money, space, tools, and mechanical aptitude to complete the swap.

Turbo Tom utilizes swap components from Flyin Miata or V8 Roadsters to do a nice clean professional job.

Agreed, watch the series of YouTube videos by Turbo Tom. They are excellent and he includes one that runs down the full list of parts he bought and costs.
I was lucky enough to buy one already done by Flyin' Miata, a 2001 with the LS376/525. Looking thru their 100+ photos of the car being built it became clear to me it involved disassembling most of the car and modifying much of it. The end result looks so stock that it's easy to underestimate how intensive the project was.
Watch Turbo Tom's videos, very helpful. Good luck with it!

Turbo Tom was my inspiration as well. Flyin Miata has a PDF guide for doing the swap, and some parts to help. Reading the guide will give you an idea if you want to jump in.