There was a similar question a few weeks ago. The answer is sort of. Total engine bay lenth of a PW is about the same as a 6B without the accessories.
I have seen the photos of a completed conversion. In my opinion it ruined the looks of the b-1. I have also seen the same conversion in a carryall (WC 53 3/4 ton Dodge).
In both cases the radiator shell was lengthened around 5 inches to get the necessary length in the engine bay. The firewall was also "doghoused" inward to help with clearance.
The radiator shell really did not look good. The look might change completely if the hood were cut and 5" welded in to the center of it, I don't know.
A front 2" lift is also needed because of the engine height.
The 4B's are a much better fit length wise, but also vibrate more. There are a couple people on the power wagon site with 4b's who post regularly and also one with a 4-Perkins.
I am thinking seriously about putting a 4B into my v-8 carrall, but have not made the financial "leap" yet.
Hope this helps.
Tim