Slant six in a Miata ? Why ???

You need to find like a mid 80s Daytona and drop a small block in it. Mopar performance made a complete K member for that swap. Of course, you have to design and install whatever rear end you choose. But those kits were pretty popular. They pop up on eBay every now and then. They fit any of that same body. Daytona, Sundance......all those same cars....."whatever" letter they were.

I put a 360 with a 300Z rear end into a 85 Lazer , got to roller stage but had to get rid of it cause of a move. When the the V6 gets too many miles on it in my 2013 Avenger RT like to swap 360 or modern hemi into it.