Here is a layout I worked on this morning based on the layout you preferred. As you can see, I moved the engine stall to the inside of the mainline. The engine stall on the outside of the mainline restricted available area to work with. The plan will need to be tweaked to make room for your larger engines to get around the engine stall, but should provide you some ideas.
Another option to consider is to raise the mainline to an upper level just for the engines that require the wider curves, and build a non-connected track plan below. The lower track plan could wind in and out of the upper track plan and give you even more real estate to work with.
