Would this be worth doing? Is it even possible or are the bore sizes of the manifolds different? With a tune what would be a good number to see gained from this if possible?

 

Put that mod waaaaaay down on your list of things to do as most people here are making a LOT of power still using the stock throttle bodys. You would need a new intake manifold to make it work as the stock manifold opening is to small to try to port match to the 90.

If you want a more cost effective throttle body mod, do a search here and on ls1tech forums for porting the stock one. It's pretty easy to accomplish and does provide good gains for the buck on other platforms, though gains haven't been documented on trucks as far as I know.