You can fit an E3D V6 but you lose some Z height as it is significantly longer and the fan duct won't fit.
There are STLs on github for an alternative Wades block that loses less height and a shallower fan duct and a different fan bracket to clear the hot end fan. [
github.com]
You also have to provide permanent 12V to the hot end fan by connecting the positive to the 12V heater connection and negative to the pin intended for a probe and the ground the spare wire at the Melzi end.
You would then have the issue that the feed path though the Wades block is for 3mm filament. It will probably work with stiff filaments as is or you could regenerate the STL with a smaller filament path, or perhaps line it with a PTFE tube. That might allow 1.75mm flexibles to be printed as well but I have never tried it on my Mendel90's.
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www.hydraraptor.blogspot.com]