Well maybe nitro express is a tad optimistic, I just wanted to differentiate from the hundreds of "smokeless 577/450" threads out there.

I was aware of the need for a solid Greener action but not the need to have looser headspace, makes sense now you mention it though.
My original plan was to build my own action from scratch for a different cartridge but thought of the Greener, and through that the 577/450, as a readily available and cheap alternative.

I think I may stick with the original plan of action and build my own. Thanks for your help and steering me back on to what is, if I am honest, my preferred path.