Alan, I’m just using a regular old line-lock from Jegs. Not specifically rated as a parking brake as I doubt any truly are as they induce and maintain a certain level of pressure in the system. This in theory can cause a fluid leak down if there is any type of leak in the system. The mechanical ones used on aircraft are suitable for leaving "set" yet can cause the same issue. With that said, airplanes are rarely left on any substantial type of incline as our cars are. I figure the line-lock will get very little use and I still question even installing it. It would take a lot to get the car rolling while left in Reverse or First with the engine compression ratio and gearing.
As far as trimming, pic below. I did not have to do any trimming for the engine to fit if I had left it all stock. However, I decided to bolt on the Stainless Headers off the 96+ S52 M3 Engine as opposed to the '95 Cast Iron, which did require a little clearance. I decided cut the inner corner out from the Batt shelf to the foot well box as I could fabricate a nice filler panel. The fit was tight at that point for the cyl head but acceptable and now I have just a little more piece of mind. I don't have any pics of what I did to fill the hole but I took a piece of 3" exhaust pipe, sliced it open and welded it in place. Made for a pretty clean / almost factory appearance.
I also decided to modify the LH inner fender well to mirror the concave pressing found on the RH for the air cleaner. This gave me a great spot to snake the cold air intake down the side of the engine and through the radiator shroud to draw from in front of the radiator. So, both my inner fender wells now look about the same.
None of the trimming I did was absolutely necessary and I was originally dead set not to change anything that would be permanent. Well, once into the project I decided I'd never be going back stock ever so why not make it just how I want.
I made absolutely no/zero changes to the steering system, rack placement or column. I did not want any issues while driving in the twistys.