Update time.
The original plan to use JB Weld to assemble the blocks has been revised after some feedback that it will become brittle over time. Instead gasket filler is the winner and together with all 16 screws in place I see no way how it will come apart with the pressure generated using normal watercooling.
First block done, the second is still curing. The holes for the barbs are threaded with 1/4" BSP. Originally th eplan was to use 3/8" barbs and tubes for these blocks but after receiving the 1/2" to 2 x 3/8" T fittings that plan was revised instantly. The ID of the 3/8" sections are just 5mm so now it will be 1/2" fittings and 3/8" tubing. It will be interesting to get the tube on but most things can be solved with a bit of violence
The TDX block is just there as a size reference...