Performance is more in the definition than the carb. I placed a B20 in mine and have been trying everything since. The Low end issue can be addressed also with a rear end ratio or axle swap. Later 544's with 4 speeds had 4.11 (I think) while earlier ones with 3 speeds had 4.56. I've made the change from H4 to H6 on the B16 and then went onto the B20 and a M41. I put on Headers and a single Weber down draught (There's one on ebay today)I didn't like the Weber at driving speeds but loved the "GOOSE" when I stomped it and the other barrel opened up. So I went back to SU's and re-jetted, replaced all moving parts and it has the performance and reliability I wanted. Nice smooth acceleration on the highway that was missing on the weber. A great distributor is required and electronic ignition keeps the spark nice. But the cam and '71 E head are perhaps the best and easiest way to improve performance. The B20 and B18 are vertually identical but for the bore and all most all the vintage racers use the B20 and they can pull near 200hp from one when tuned to the top limits.