Your only real option is to buy say an unfinished Lancair kit and go much bigger on the engine and turbo and start doing any drag reduction you can do. You would have to completely evaluate the airframe to match VNE to your cruise goal plus a safety factor. If you got your hands on a kit that didn't have the wings closed you could do what was necessary to get it to be safe at the higher cruise speed. There are people modifying and racing these at Reno and getting the speeds you want so there are people to talk to to find out what, if any, mods were done to handle more power and more aero forces especially drag and increased low pressure zones on the wings. It would be a lot of math to do justifications but it can be done. I am not sure what airfoil the Lancair uses but reflex could reduce drag significantly at high speed cruise.
Anyway just a thought. You don't have a lot of experimental options other than clean slate really. Clean slate would be A small low drag wing and a small sleek fuselage with a big turbo normalized engine and flower or some sort of advanced flap design like the Melmoth 2. Peter Garrison has done a lot of the homework for the design you are talking about but stopped short of your cruise goal.