I have someone here with a 68 Barracuda, slant six 4-speed OD (not stock obviously). He wants to buy my clutch kit (for a 130T flywheel, SB V8) and is wondering if it'll fit, or if he can make it fit. Google is not terribly useful here, listing everything from 122T to 130T to 143T flywheels. I'm guessing that if he finds something other than a 130T flywheel in there (he doesn't know yet, will be taking it apart soon) then he can just swap it out for a common V8 130T one?

An original slant 6 clutch would have been a 9 1/2". Likely, your clutch is a 10 1/2" so if the 68 Barracuda still has the original slant 6 clutch, yours won't work for him.
That said, around that time Mopar came out with a heavy duty 10 1/2" clutch set up for light duty pickups. Its not very likely that HD clutch & bell would fit in the Barracuda, but one never knows what people have done in the 60 + years since the Barracuda left the factory. Gene

Car Slant uses a 122 tooth flywheel. The biggest clutch it will take is a 10". 130 tooth flywheel will not fit in the bellhousing.

I had an adapter plate made so I could use a 130 t flywheel and small block bellhousing, but I sold it years ago.

His tranny is out ov a newer truck though, its a 4-speed 833 OD. Would they not have a bellhousing that fits a 130T wheel? The trucks and vans i think had the bigger wheels, later on?

My clutch is a 130T 10.5" one.

All of the NP833 trannys I've seen all have the same bolt pattern to the bell housings, all the way from 1964 to 1980 or later on the O.D. NP833 cases
Sorry to be the bearer of bad news
He will need to find a bellhousing for a 10.5 inch clutch and a 130 tooth flywheel for the slant six motor to use your clutch kit

Trucks used the "HD" 10" clutch with an 833. The bigger clutches used a really funky bellhousing that will neither fit in a car nor accept an 833.

Hmmm... does not look good then. Damn.