Top speed (or gas mileage while cruising in top gear) wouldn't be affected by a transmission swap because the top gear is always 1:1.

What's different is the acceleration from a stop if 1st gear is used - BIG difference then. If you don't put a 4-speed AT into 1st, though, it'll automatically start in 2nd, making acceleration about equal to the '72 450SL with 3-speed AT.