Similar thread last week, similar answers.
Have some ideas for a build up... suggestions?
Biggest difference is that with your engine you can run a roller. I really like the Crane 2030, absolutely massive midrange. If you do the build up using the stock heads or if you tow a lot, the 2020 might be more to your liking.
Trying to roll 38" rubber with a 5.0, any 5.0, is going to be a bit of a challenge. That's a whole lot of polar inertia you need to overcome and even a well built 5.0 isn't going to be a torque monster.
You also need to think about your clutch and tranny. The Mazda is a weak piece at best and you're looking at increasing the input loads substantially. you've already increased the output side loads and something's going to act as a fuse somewhere in the powertrain; I'd bet on the tranny. I'd be looking to upgrade to a ZF if it were mine.
The OBD-II stuff isn't nearly as harmful as you think, it just forces you to keep the engine emissions compliant which you can easily do. There are huge quantities of "green" speed parts available for the 5.0.