If you weren't dead set on propane, maybe you could find an 8.1 w/Allison out of a wrecked 2500/3500 from 2001 to whenever it was dropped, possibly 2006. It would definitely out perform 2 old GMC v6 gas engines, if that's what a 401M is.
I just remembered another truck I have actually driven with an 8.1 & Allison trans. It was an '05 GMC 6500 U-Haul truck. That thing was a hotrod. It could probably be converted to propane or CNG. This engine/trans should be available from wrecking yards.