Well, it can be done, but you are better off just finding a 70s van and rebuilding it. The sheetmetal is the same from the bottom of the winshield back, but there are very significant differences forward to the front. In addition to removing all the sheetmetal forward of the firewall, the front suspension and K frame changed in 79. Seriously, if you want a 70s van, just get a 70s van. It is less work to completely rebuild a clapped out 70s van than it would be to swap a 70s front end onto a 79-up van.
That said, get the 2003 van and customize it like you want. Try it, you'll like it!