going from 4.56 to 4.10 gears didnt hurt anything much for me at the track. a well calibrated carb should run as good as fuel injection at a fraction of the cost. if i were you, i would go for a 750 double pumper. make it work good on your primaries on the street (jetting, power valve and accelerator pump) then tune the secondaries at the track. for your converter, 3500-3800 will be enough. one thing i learned with the 557 cam is that it likes a lot of initial advance (timing). a locked distributor (no vacuum and no mechanical advance) should work at that level. i just loved the rumble of this cam!
