I went with the Gaui X5 flybar mainshaft because it was the longest 8mm shaft I knew of. The reason for this was to maintain a scale rotor height, while maintaining the ~5 degree forward main rotor angle like the real thing has (need the airframe lower to create this effect). You do have to get that origional HK500 autorotation bearing sleeve which was out of spec with the Align part to adapt the Gaui shaft to the Align main gear assy (8mm straight through where the Align part allows for that stepped main shaft). I did check the HK site recently to see if they still sell that, it is off the site now when I checked. Before it was listed as just out of stock, and the comments/ratings section had a bunch of people commenting how that part was out of spec. I guess you might be able to get one from someone unloading an origional HK500 kit, or maybe some other clone supplier is still selling that non-updated clone part.
My fuselage origionally had a T550 in it, and that was mounted directly on the floor of the fuselage. To beef up the floor, I layed in a couple layers of fiberglass and resin. Later I put the stretch in it, so to do this I made some blocks out of some 5/8" pine board I just happened to have around and then glued those to the floor of the fuselage. I have some ply board screwed to the base of the airframe, and that board screws to the blocks. Somewhat crude, but it works, lol. I also have the HK aluminum airframe base because my origional was stripped out where the nuts go. Maybe that stiffens things up a bit, maybe not. Starting from scratch I think I'd just glue a thin sheet of ply onto the floor of the fuselage and then do the same block routine I'm doing now. I did the fiberglass route before as I was in a crunch for thinness working with the T550 in the fuselage.
I also made some custom links out of some threaded linkage rod bits JBweld'ed into some CF tubing to account for the higher main rotor height. Yep, I still need to paint that aftermarket head block to match