I run a 350, turbo 400 (Auto) & Dana 20, pretty much the same as a Dana 300 & both cases can be twin sticked for less than $50.  I like the auto tranny mostly because I don't like the hassle of a stick anymore.  If I was doing it over I would go with a 203-205 doubler my front drive shaft is short & at a bad angle the doubler would give it more length.  I am not sure about the strength of a toy tranny & t case behind the 4.3 or V-8 especially with those size tires.