what size carb ?

If you have the original 340 intake, use it. There's not ONE SINGLE aftermarket aluminum spread bore intake that can outflow it. NOT ONE. Well, ok, maybe one. The Offenhauser, but it's like 350 bucks. Those intakes flowed way better than they led on. You can actually do a bit better than your 73 model. The 1971 340 intake is the best, because it did not even have the EGR port bulges in the plenum. The 1972 had the bulges, while the 1973 was actually machined for the EGR passages, but came from the factory with plugs in the plenum holes. Since the 71 has a completely flat plenum bottom, it flows the best of the three.