350 = bad idea in a Miata because the distributor is at the back. 302 doesn't have that problem, distributor is in the front, letting you shove the engine farther back.

When it comes down to it, the Miata was built to be perfectly balanced with the engine it's got. Swapping anything else into it corrupts the feel of the car, to me anyway.

Despite that, I recently started on a V6 swap into my second Miata, but aborted it in favor of something cheaper and much, much faster.

It would have worked out quite well though. Ford 3.8 V6/Borg Warner T-5 tranny, it all fit very nicely...