Hi Lui,
After your post here, it had me thinking. Sorry to have posted the 4.6 engine install info, I didn't know your car was set up with a mod motor already.
I did do some research, and finding a 5.8T for sale was more difficult than I thought it would be. I only found one source for them, it seems like Ford does not list them, and from what I was told you do have to "have connections?" to get them from Ford. I did see Ford has the block in aluminum available, and since the Shelby's block use's a spray welded liner. I would consider that a throw away block.
Here is the source for the 5.8T
5.8T for sale. The rep there told me the 5.8 does have the same dimensions as the 5.4 mod motors. But at $31k US for the motor, plus the modifications needed to set the motor back.
Hope this helps.
Jeff