Yes the left head would need to come off to change the distributor gear.

My car is an 86 Euro 500SE with a 86 Euro 420 engine and a 722 311 transmission. The 116 420 engine would be a direct bolt in but you would probaby have to use a a 420 ECU ,Steve would know.as my car came with a 117 engine I think that a 500 or 560 would be a straight bolt in but you might need a different tranny. I bought my car sight unseen as a 500 and was P------ off when I found out it had a 420 engine but after almost two years of driving I really like the 420 engine as the car is a Euro model it is geared very high so I'm getting a little over 20MPG with lots of 2 lane Mt. driving............
William Rogers.......