Not sure about smallest bellhousing, but I purchased one (to use with T-5 Ford) for my Lotus Elan project, and Elans have modest sized housings. You might check others at rwdmotorsports.com. There's a company or two out there that makes custom bell housings from spun steel. Sorry I don't remember names.
5" to 7" racing type clutches are typically multi disc and can handle hundreds of pounds feet of torque. And they can be adapted to most any size flywheel. The small diameter is about reducing rotational inertia, not limiting torque capacity. Keep in mind racing type clutches tend to have quick engagement characteristics, i.e. 'grabby'.
For smooth engagement, you need a street clutch disc, and to get the torque capacity you are likely looking at 7.5" or larger disc with an appropriately rated pressure plate.
For multi disc see for example:https://www.speedwaymotors.com/Ram-Clutches-9371S-Disc-Clutch-Chevy-Triple-Disc-10-Spline-1986-Up,49600.html
I would be looking at performance clutches for Formula Ford or BMC A series engines or similar. Small, ubiquitous, so not too exotic or expensive.
