I have a 74 MGB GT with a Rover V8 in it and I have measured for the LS1. The LS1 in larger then the Rover V8 in only the width from head to head at just over 1/2 an inch. here is a pic of the Rover block and the LS1 sitting next to each other.
here are the engine bay pics of my car.
with the radiator moved formard there is some room up there, but that is not going to be the problem with putting the LS1 in there. the real problem will be getting the crank pulley to clear the sway bar, to get the rover to fit past the swap bar the harmonic ballencer has to have all the pulleys removed from it and machine a V-belt grove in the balencer I'm not sure how you would do this on the LS1 if it was needed. I have not test fit the LS1 in there but it does look like it will be a close fit to that sway bar.
But here are somethings I have been thinking about with the MGB. The Rover motor is a light motor weighing in at 327 pounds, even the LS1 is not that light. Also being that I have the Rover V8 in now I want to use the same type header and exhaust arangement, with the RV8 stlye headers that go out the inner fender. Also there really is no room to have a block hugger style header any bigger then a 1" 5/8 and that would even been a tight fit I'm sure with shitty flow. there is more but it is late and I got to get some sleep I will post more later.
Also as far as the T56 in the MGB I have seen one behind a 4.9L stroker Rover motor little hammer time on the tunnel and it is in there.