Solid roller cam
Sheetmetal tunnel ram
Much higher compression ratio because you can use better fuel
55 more cubes
Drag car can be built to use narrower TQ curve witch can allow you to build a higher peak HP number. Still with all that 300 more hp is a tall order but I think it could be done, and yes you will need lots of valve springs.
Mabey you could mix your own e-85 with 15% 115 octane gasoline and 85% ethanol and that could add a few more HP by bringing in a little more oxygen than is possible with pure gasoline.