I'm not sure what you mean by "has this engine/tranny combo been tried before"?
It was used in S-10's and some half-tons.
This combo should get you decent mileage, providing the rear tire size and diff give you a good final gear ratio. It should work just fine.
Do you have any photos of your GMC? I'm also building a 46 GMC and I'm in the Edmonton area.
Last edited by 46gmcpu; 07/05/2012 3:27 PM.