Get an external fluid Reservoir like this: https://www.summitracing.com/parts/pfn-sr146h610sa
That's just the first one I was able to quickly find doing a search on summit, but it has 3 ports specifically for running HB. So you could use that and do the low pressure return from the rack and the HB to that reservoir and than the main return from the external reservoir back to the power steering pump. Putting a small cooler inline would help everything too.
What size is your master cylinder? And are you dead set on running HB? I know you're already pretty deep into this, but changing up to manual brakes or if your cam will allow it, just running a regular brake booster could save you additional headaches.
I'm also worried that the U-joint making that almost 90* bend is going to bind and give you steering trouble down the line. The shop I use to work at put that same unisteer setup on a 69 camaro and we had a binding/clicking issue in the steering that took forever to resolve.