Rebuilding Two Stage Cascade Phase Change Cooler

[LordNicoll]

Hello,
Recently I was very lucky to be given a two stage cascade unit by the overclocker Unseen. He mentioned he was very active here in the day so I decided here might be a good place to start asking questions, or more accurately looking for advice.

The unit in it's current forms has R404A in both stages, Unseen mentioned he was unable to get something like R1150 for the second stage, a problem I do not have. I have some welding I want to do on the aluminium chassis it's mounted to, and adding some heavy duty coasters and then some rework to do on the stainless steel cover (namely routing the eval line out the top not side of the unit). Most of my questions pertain to charging a cascade unit, do I treat the unit just like a normal phase change unit when charging, just one stage at a time? Whats the superheat settings I should be aiming for? Talking a bit with Bartek, I'll probably replace the evap head with one of his.

I am rebuilding this unit up for a very high load capability not lowest temperature possible, so he recommended a shortened capillary tube, currently it's pretty long. I also noted the lack of an oil return line and oil separator, is that something that is needed or is it's omission normal? I plan to use R404A and R1150 in the unit, first and second stage respectfully, but are there better options?

I've never really actually torn a phase change unit down with the intention of rebuilding it, so I'm a bit green when it comes to this. Any advice would be welcomed beyond "Don't try this it's very hard" I included a photo of the unit from the top.

Oh, another bit of pertinent information, the high heat load could be an i9 7900X, I do need a permanent GPU bench to quickly bench GPUs so around a 5.4-5.5GHz i9 would be nice, so I'd hope the unit can maintain a clock that, I'm confident it can.