Swap out engine with tranny attached or not?

I am switching out the old 273 for a rebuilt 318 in my 65 dart. Any comments as to easier to leave the tranny attached or removed during the swap out and back in?

What year is the 318? If it is 1968 or newer you will need a bushing for the crankshaft because the hub on the convertor will be smaller if it is original to the car.