The GTD factory used to offer the 750 cfm vacuum secondary carb as standard. If you wanted the DP version, they offered the 650 cfm DP as from their experience that is what worked well on the typical GTD setup. According to the Factory! However I have the 650 DP on my car and have no complaints. A 600cfm is likely to be ok if your engine is not massively powerful or you do not intend to sit at high rpm all the time. I ran a 600DP at the Ring in 1994 and ran out of puff at 145 mph although partly due to small primary exhaust bore. It is all about breathing your engine.