I build these sets all of the time. With a 340 engine you will need a 1.08 or 1.14 center and two 1.14 out boards max. Anything larger will cause issues with driveability. You will need to find a manifold that uses the Holley 2300 bolt pattern. You can use the 289 setup that Ford used on the tri powers that were installed at the dealer. It is sold on E-bay and is common. The fuel can come from a fuel log or you can get creative and make your own. They are very easy to set up and once you have them tuned right you won't have to touch them. They use progressive linkage so you are running off the center carb until 1/2 throttle. You can get up to 16mpg with a properly set up tri power. Here is my website for more info. http://mysite.verizon.net/vze2gsgu/.
Kurt
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HAVE A NICE DAY!!!!