go with the 750 for that big motor.Be abslutely sure that the timing is correct at idle and correct at full advance.34% at 3400 rpms is a must.You can alter the idle timing all you want but you must get the total advance all the way 'in' by 32-3400 rpms.Those motors all run 'muddy' anyway.So get the timing right before you finalize carb adjustments.Higher idle speeds help keep those big motors happy,but not to the point of drive-line damage.800-900 ok in netual. good luck