Thanks a lot! Don't forget to show us your solution when ready!
Ok, so I had some time to bench the CPU part while waiting for the GPU copper block to cure...
But first, I wanted to check how good was the contacts of my setup with the help of surface pressure films from SENSOR PRODUCTS INC.

This is what I got when placing the pressure film in between the CPU IHS and the Streacom HT4 heatpipes:

The red areas show where there is a good contact pressure. Disappointing isn't it?
So I took another pressure film which is designed for lower pressure levels, thinking I would get a better result:

Nothing better, only more noise...
Let's see how it is in between the HT4 and the aluminium heatsink:

Not good either, only a few heatpipes seems to touch properly the heatsink.
Both results here with the two sides of the pressure films:

(HT4 films should be rotated 90° to be consistent with the CPU result)
I anyway went on with that, hoping the thermal paste will improve the situation.
Some Coollaboratory Liquid Copper thermal paste on the CPU IHS:

And a bit of EK Ectotherm on the side of the HT4 that is going to be applied against the aluminium heatsink:

All that in a computer case that is not just a frame anymore!
Plexi side panels have indeed been added, I took them seethrough so that I will be able to make some air circulation test later.
[MEDIA=imgur]a/gT1KxEF[/MEDIA]
Will show you the temp results in the next post!