Advance Adapter no longer lists an adapter plate, that was the first site I checked to come up with an answer. They also do not make a bellhousing to fit the big V6.
The Getrag won't work with the V6 either, bolt pattern is wrong on the integral bellhousing and that trans is far too light for a 1 ton truck.
One option that might be considered, though, would be to use a Ranger Overdrive from Advance Adapters. No cheap, but it will bolt up to the existing bellhousing and the SM420 trans. Plus it will give 8 forward gears plus 2 reverse.
