I was doing some header/exhaust fitment the other day, first time I actually noticed that the 13b header is on the opposite side as the 1.6l. I fit the large boxy part of the 13b's header after I grinded down the corner on the lowered "fin" of it where it was touching my subframe. It seems to come down in a good spot but there will definitely need to be a custom section made to link the headers to my Miata Borla cat-back system. I'm just going to get car running on open header first before making the custom section in case things need to change. My passenger side mount I made a plate that used the same bolt pattern to go under the oil pan (where that cast aluminum bracket went) and then welded a plate coming off of my tubular subframe, then I used a Chevy S10 suspension bushing (control arm I think) as my rubber shock obsorber between the 2 plates, it's all connected with a bolt and washer. Driver side is the same setup besides it uses and L shaped plate that bolted to the center Iron. Then I have a hybrid transmission using the stock Miata tailshaft and mid gear section, and the RX7 bell housing so the rear is supported by the PPF in the factory position. Honestly I have no idea how my engine mounts will work when the car runs, it was my first time ever making something like that so I'm sure there is a better way to do it. The engine is very stiff though, when I push and pull it barely moves, compared to my Miata engine which moved about 1.5 inches either way.