..I'm sure you will get more than one opinion on your question. Here is my take on it. Follow closly to what i'm trying to put across. The 750 will work at some loss of throttle response due to the larger venturis that only help at wide open throttle and higher RPM.
..The engine (302) is only capable of drawing about 500 cfm +/- of air at 6000 rpm for 100% volumetric efficency. Your engine's efficiency will only be near 70-80% at wide open throttle.
..That said,the 750 and the 600 both when setup with a springs to allow the secondary to tip in (at the correct point) would not fully open.
..The 750 would have a very small advantage at wide open throttle due to allowing air to pass with less restriction and contributing to the loss in volumetric efficiency along with the manifold,intake track, head port and valve losses.
..We don't run our engines like that just going to the store so the smaller unit would be the choice. Even better would be a spreadbore carb but it requires a different intake. Those carbs are quite expensive. They have small primary-large secondary so you have the best of both response and high RPM operation.
..Tried to give the scoop on this as the question is often asked. Again if the secondary is set up to come in at the right time,a vaccuum carb can't overcarb the engine but starting with a larger unit will tend to degrade throttle response.
..Good luck ,Ken