Don't worry, I'm still enjoying it at the moment and it's helping to sharpen some skills that have started to dull since I changed jobs. I'll only start to charge when I start to loose patience with it
I'd thought the IP gearbox might be a good choice too. I had worried that maybe it wouldn't be butch enough but in reality it shouldn't have too much work to do. I think I can make a pretty good representation of it from the measurements on the website. Once the floor is in I'll have a go at trying to get it in there.
I have a couple of the IP chassis in 2 of my Steam Trams. I can manage a 2D drawing for you to scale if it helps. I use MacDraft PE on my iMac, I doubt the files would be compatible for you but at least they will be scale size.