I used a V-Mach BrumBow one in mine but only because I had a few! You might like to investigate HW85/95/98; Haenel 303 or Titan #7 springs! You never know, someone may have such a thing kicking around and could help out!
If; when you have it sorted and tested for MV and ME and variations; I could help you to refine the innards to get the values desired (assuming you were not there already!). I would urge using a custom main guide and Top-Hat assembly though; makes all the difference in eliminating the internal cacophony! Whether it's worth all that effort; only you can decide!
atvb
David