And we know from the evidence that Oswald was the person responsible. It was his weapon, his shells, his fragments of bullets recovered from the limo.
So there is no need to alter the body. Lifton painted himself into a corner - and he explains exactly how in his book - by refusing to accept what the evidence was telling him -- nay, screaming out to him. And Lifton then invented a elaborate shell game to retain his beliefs... that all the evidence, including the President's body, was altered to frame Oswald for shooting the President from behind.
Why go to the elaborate nonsense of shooting the President from the front and then altering the body? And planting a gun, and shells, and fragments, and a nearly whole bullet at Parkland? And then plant Oswald's prints on the rifle later? All this is part of Lifton's bizarre theory.
Why not just shoot the President from behind using Oswald's gun?
Why didn't the conspirators think of that?
Wouldn't that have been much simpler?
Hank