IMO a mech. sec. is out of place on the street unless its pro-street with serious mods. If the 750 was vac. I'd say tune it. It would work great. Low speed torque and hi speed horses. The reason I think you dropped so much power is that you have power valve problems, way too small jets, or too strong of vac. spring. Either buy a vac. 750 or tune the 600 The 750 tuned right will make more power. You can tune it yourself, just takes a little tinkering.