I would think it would depend more on the condition of the LE and whether or not it is a “true” LE. I’ll leave that to the experts here.
While the later Rubber Bumper B’s 77 to 80 are probably the best choice, for a Buick/Rover V8 conversion (or any conversion for that matter) as the engine bay is already pretty much setup to accept that V8 with little or no cutting or modifications.
But where you can have problems is with emissions. Here in Kalifornia (76 onward) as well as a number of other states, most Rubber Bumper Bs still have to be smog tested. While not impossible to do even with all of the strict & difficult hoops you’d have to jump through with a state appointed referee just to get it registered. Perhaps an earlier B even with all the cutting & modifications might be the better choice.
My $0.02,
Bret