Put a set on a friends real Hi-Po but faux '65 GT350. The right side header seems to be the one most vendors get wrong, but the Hedman product had the correct bends to clear the spring tower. '65 Hi-Po's have a different motor mount than the other '65 289 cars, and I don't know if that makes a difference on header installation.
66 mounts raise the engine a bit. And you're right about right side Tri-Y problem. I had problems where the primaries of cyl 1 and 2 connect. That hit the shock tower and seam weld is pretty hard to dimple.