I wouldn't worry about getting the pin centered over the axles.
Why not?
You have 40K rears. Legally able to put 34K on them. Your front is legal up to 12.5K, depending upon the axle and tire specs.
Once you build out the box, your front will likely be pushing up against legal limits anyway. Putting the pin weight from the goose at the rear will alleviate a lot of the weight concerns over the front axle. If your wheelbase is, say, 264, and the pin weight is centered 132" behind the axle center, you will be losing 1 lb up front for every 2 lb of pin weight out back. If that goose generally pushes down about 4600 on the pin, you gain another 2300 in capacity up front. Its not like you're ever going to overload the rear of the thing, the front axle will always be your concern.
If this becomes an issue in the future, you can always eliminate the front drive and run rear only. This will shift the axle center rearward and transfer more to the front. I'd do this anyway if you weren't planning on coming in over 32K from the truck alone.