Don't the Dmax engines have aluminum heads?
Something tells me that these weigh in between 800 - 900 Lbs.
Also, those 60's trucks tended to have low ratio differentials.

If you can get a Dmax with the 6 speed Allison, it will set you back a bit (OK, a lot). but It is a great engine and the 6 speed Allison has a .62 top gear making whatever rear end you have very usable for normal driving.