Very interested in what you guys are doing.

I've also been collecting 5 speed MX5 boxes with the same view. Not a light box though, about 10kgs heavier than the series 60 box. I haven't done anything yet but have an auto bell housing that could be welded to the Mazda case.

I also wonder if bell part of the case could be milled where one of the two rings circles the entire case. Doing so and making a 3mm steel ring that bolted on with a lot of 6mm bolts similar to how three piece wheels are made. That could allow a range of steel mini bell housings being made for various engines. With ribbing and using 2.5 or 3mm steel the fabricated adapters could still be light.

With a steel mini housing, centralizing the input shaft to the pilot bearing would be simple. Two oversize holes with precise washers on opposite sides of the block, dial in the centre and a quick tack weld of the washers does the job.

Does anyone know who makes pressed steel cones that can be converted into bellousings? They are used in the USA in the making of larger adapters for heavy v8s.