The pin area is probably the least restrictive part. if you assume that the gap between the pins is 2mm then it would equal to 14mm across the pinned area. I would assume this is wider than the channel. input/output area will create the most restriction imo.
The front block channel looks similar to mine other than I never finished off the groves to pins due to snapping all my 2mm bits
You could also mill out a triangle on the bottom part of the backside block to reduce weight without it being part of the channeling. Also a mica shim, (or shims), in the backside gpu area to help transfer some heat to the backside block.
The perspex tube tidy is a nice idea and would look good, but making the 90 degree output/inputs curve inside each other and keeping it all straight and true will be tricky to do well.