I started with MX5 seats and as they are ADR'd you can use them.
In fact I narrowed mine 50mm to better fit the G-Force cockpit and had the leather skins professionally downsized so they looked the goods but forgot to tell the engineer all that.
These still didn't work for me, so after engineering I had Marty Govett at Kenmer make up a pair of seat frames he's had ADR approved for his kits.
I ended up with a narrowed version of his normal frame which I had upholstered into a traditional look seat, as shown below.
Raw frame in-situ
Traditional look non-adjustable bucket
The whole shooting match trimmed
These are a comfy seat I've had no issues with on long drives but this would also be related to the quality of the foam bolstering used.
Marty's down here in Victoria but any thing is do-able.
Good luck on the research!