I just want to throw this out there because I dealt with something a little similar when I swapped my 3.9 for a 5.9 in my 02.SWB.
Just for info sake, the swap was seamless. Everything bolted up except the driveshaft. Since there was no SWB V8s I had to have a driveshaft made for the 4sp OD trans.
We did have to redo the engine and trans. harness. Still we couldnt get the indicator light for the transmission to work. We even swapped clusters too. This is the only issue I had from the swap. Now the transmission still goes into gear and even into OD, but i do not have the OD cancel function nor the button on the stalk. We did some quick diagnosis and we found the wiring harness for the steering column is different for the V8 vs the V6. Someday Ill put it in (along with the V8 transmission stalk):but today, it really doesnt matter to me. I do have the whole steering column from the donor so someday Ill do it... The van should have first come with a V8 :cheers:

 

Update: Well,... it took forever, and I certainly have learned a few things. As I mentioned above I have been having issues with the voltage regulator which is in the computer. That along with some false trouble codes, the fuel guage not working properly, and so forth. So, I finally bit the bullet and ordered in a reman computer from autocomputerexchange.com. I explained to their main man Louis my issues with the cruise control. Long story short, I received the computer today and installed it. All the gauges are working, so I took it out on the road to see if the charge gauge would go wonky or not. It didn't. Also, while on the road, I decided to give the ol' cruise button a push, and to my disappointment, the little green light didn't come on. Now, I've pulled the dashboard to check, and there is indeed a lightbulb back there in the socket. But,...alas, no green light. But I'm stubborn, so I hit the button again and toggled to the "set" button. Still no green light,... but then I felt it. The gas pedal dipped and the van took over. The cruise control works. So, one way or another, it was a computer issue.

As for the light. I pulled the cluster again, and double checked the bulb. I swapped it with the O/D off bulb, and found that the bulb is good. I noticed there are contacts missing in the plugs, so perhaps there is a wire that isn't installed for the bulb. I have to do some research and see if the vans that have cruise have extra wires going to those plugs. If so, I imagine I can pull those wiring harnesses from one at a Pick-n-Pull. I'll update this thread when I find out. In the meantime, thanks to everyone for your help and advice.