Interesting - I have been told that a more suitable box good to go for these days that is not hard to source is -
Nissan 350Z model dates between 2003 to 2006
Trans - 6 speed VQ35 / cdo#1
First Gear Ratio (:1):3.79
Second Gear Ratio (:1):2.32
Third Gear Ratio (:1):1.62
Fourth Gear Ratio (:1):1.27
Fifth Gear Ratio (:1):1.00
Sixth Gear Ratio (:1):0.79
Reverse Ratio (:1):3.45
Final Drive Axle Ratio (:1):3.54
Perhaps worth investigating if your keen to explore a swap, but again all the issues of custom work required
I actually have one of those transmissions at the shop right now, but it is slated to go with the VQ35 into a 280Z, which I wish would have been left the way it was.
I recall at some point seeing a bellhousing on eBay for that very box, but I don't believe it's out there anymore. It was meant for the M117/M119.
Again, the bellhousing is a rather simple prospect, it's the flywheel that's an issue. Driveshafts can be measured for and made fairly inexpensively.
I've read over on PP about using a Ford Modular flywheel with the ring gear from a 3.5 affixed to it, but I believe that's for use with a T5.
A company called Rennsport in the UK offers a Flywheel and bellhousing kit for the C230K 6-speed for the M117.
I am actually going to email them again and see if the flywheel would be a good fit for the TKO500, in which case I'd simply purchase their flywheel and get an appropriate clutch kit and go from there.
Just to further the proof of concept, here is the T5 in the MSM Miata with a custom aluminum crossmember that we've since redesigned:
The car ran and drove great, and the 5-speed ratios are better suited to the car than the factory 6-speed. The OD ratio really brings cruising RPMs down significantly at speed, which makes for a more quiet and smooth ride.
Unfortunately, the rear end on this MSM was on it's way out and has subsequently given up the ghost! A replacement has been sourced but we haven't swapped it in yet.