Is it possible to reflash a computer from a v6 to work with a v8 and a new transmission from a different model year.

 

For all practical purposes NO..
You should get the PCM and wiring harness from the donor (one you are getting the engine and transmission from) vehicle...

 

You MUST use the PCM that matches the transmission that is going to be used in the vehicle - the ONLY exception to that rule is if you are going from an automatic to a manual.

Also - if you change from a V6 to a V8, obviously you are going to need a different engine harness, and thus you need to use a matching PCM - or at the least, a PCM from a V8 with the exact same transmission you are going to be using. We have, for example, tuned these F-150's for 4.6 to 5.4 V8 swaps, using the same transmission and original PCM.

However, the long and short of it is this - the only smart and correct way to do this is to use the complete powertrain, wiring harness & PCM that matches the new engine & transmission in these vehicles - that way it can run on it's stock tune if need be, or run on a custom tune if you prefer - as well as the fact that this is the way to make everything work correctly.