The bell in my truck is a #272 which is the NP435 bolt pattern, 5.125" retainer opening.

There is a #670 bellhousing out there that came behind big block trucks in 77'-78'. There were no factory BB truck/833 combos to my knowledge but for whatever reason the bells were drilled for both the 833 and NP435.

I search daily for a #670 BB dual bolt pattern bellhousing. Not many out there.

Was wondering the same thing about the American Powertrain bell. If you use that I guess you are using a hydraulic clutch setup?