Ok for the life of me i dont understand what the issue is.
A little back ground info.
04 silverado rcsb 4.3 5 speed truck
base model ac cruise and rubber floor mats
Nothing special
I originally sold the 4.3 and swapped to a 454
IT was the vortec 454 so i plugged in the crank sensor so the factory pcm would be aware the motor was running and the ac would work.
Drove it for about a year, sold 454
Bought an 8.1
After doing some research it was cheaper to go the same route where i used another ecm to run the motor and leave the stock pcm in place to keep the truck stuff working correctly.
I made the mistake of putting a same year v8 vin in the truck so the tach would read correctly.

Side effects of this was a continual error message about reduced power because in o4 everything with a v8 had fly by wire throttlebody. So since the tac module wasnt installed and the electric gas pedal etc. I got this error message.

Also now the gas guage is backwards.
So i have been dealing with the issue til i had time to tinker with it some more.

Well recently i decided. I will just put the stock vin program back in it and then the only problem i would have is the tach reading being off. Now I went to the GMC dealership i used to work at to do it. So of course it gives me the latest tune to load into the pcm.
so load the tune. and now the truck wont engage the starter for more then a split second. And if i bypass the starter relay. (since the computer controls the starter circuit) The gauges do some wierd **** and all the lights go crazy on the cluster.
Wasnt sure if i just loaded the tune wrong or what
So finally after fighting with it for half of a day. I decided to just throw the tune i had in it prior. And now it starts fine and is back to the normal issues i was having.

Any one have any ideas on this one?
I know the v6 v8 pcm's are the same it is just whatever tune is loaded in it. But by loading the v8 tune in it did i reconfigure it so i cant go back to a v6 tune or something hidden that i dont know about? any help would be appreciated. I am tempted to go buy a v6 04 pcm and install it to see if that was the issue or not