Use a 1750XL and not a 1750 in the Ramlink. Smart choice going to the 128D. I'm sure you saw it in one of the old Commodore World magazines but a guy in Florida did an excellent job building a tower such as you are. There were pictures and everything. If it was me I wouldn't worry about the 5.25 inch drive at all. I would mount an FD drive internally and go only with 3.5 inch floppies. There is not very much left in the commodore software world that hasn't had the protection taken out so that it can be put on the smaller disks and also onto your CMD hard-drive. If you really need a bigger drive for something just plug it in the back. Out of the front of the tower you will want the FD drive, HD, RamLink controls, and accelerator. I had a kit that was made at one time for running a cable from the ramlink to an external control box. It was made so that you didn't have to reach to the back of your 128D to push a ramlink button. They stopped getting used much because people started setting the 128D's sideways so that the controls to the things plugged in the back could be reached.
Oh yeah, I almost forgot. I had asked you once before and I don't think I ever got your response. How are you planning to overcome the video output limitations of the 64/128. I don't think it was ever done or can be done. I know you can soeed up the processor, load instantly with ramlink, have enormous storage with the HD, but in the end you are still stuck with the crummy video output. That was why after almost 20 years I left the 8 bit scene. You run into a brick wall when it comes to getting decent looking graphics. That was why I chose to stick with the Amiga. Good luck on your project. I have to admit thinking about it makes me miss my 128D a little, but not enough to get it out of the attic.
Dan
:-D