Sounds like he's going to weld a B/RB bellhousing onto a 518
You may consider using a A618/47RH. It can handle more power than the A518. I would assume you could weld on the ultrabell since it to is aluminum(along with the bolts). I believe the pump bolt pattern is the same between the 518 and 47 so the ultrabell would be an easier install. I see they make a adapter plate to install a ZF8 to a big block, That would be even better than any 518/47rh but would need a controller to run it.
link Difference from a 518 to a 618, other than the bellhousing size, is the internals mainly in the clutch/steel count of the OD section, which can easily be built to 618/47R* spec... you would not want to run the diesel spec valvebody behind a gas engine .
