Yea, well i've got a few things that should arrive in the morning that i'm going to try out. Some standard generic white paste (TBH it works pretty much as well as AC3 is used correctly) some Akasa ShinEtsu Thermal Interface Pads which will mould itself to the gpu and and the hsf.
Also the standard plastic push pins that hold the gpu hsf normally will be scrapped. These work fine as the standard pink stuff holds the hsf very tight and it never moved. I had to loop in 3dmark for over an hour to get it to move so I could remove it. I've got some 2mm, 2.5mm and 3mm screws and wing nuts so I can be sure its not going to move.
I see around the core are 4 holes (probably for flowfx coolign system) the standard hsf uses 2 others which pisses me off as it must be tough to keep pressure even and ensure good contact without a very sticky pink thing between the core and the hsf.
So my options are to drill into the stock hsf so i can use 4 screws to hold it or get a block of copper and make my own hsf. The bottom line is I have a 5900 ultra 256 that I don't mind screwing up so I will be trying a number of things. One thing i've been itching to do is make custom hsf for front and back. If that does happen it will be in tutorial form so people can see what I did and how