Yeah, whatever goes in it will be stock, i dont like tampering with oem internals in anything.
The reason im concerned about the larger 70-75 swap is the extra parts and steps to take, where as a 45-50 is nearly bolt on.
My car on the scales is 4290 with half tank of fuel, so over 4300 with a full tank and 4500 with driver. Yes, it has pano, all 535 tourings do. So yeah, not light.
Do we have a limit to newest model year we can use the 8hp50 from? they made the 335i GT with n55 into 2017, thats a damn near new transmission and with it being almost bolt on, might be worth a shot and see what happens.
Is yours 600lbft at the crank or wheels? Im about the same to the wheels.