On a street engine, a balance tube between each carb is necessary for smooth lower speed running.You can make 3 individual small manifolds and hook them together with like one inch hose or you can fab up a log type manifold. Stainless will probably be easier to work with, the tubing is readily available.Keeping the runners short generally aids power and response in the 1500-3000 rpm range, where a street engine operates most of the time.In addition the short runners have better airflow,keeping the fuel air mix atomized, especially if you don't heat the manifold.I think heat won't be necessary for your situation, and if you need heat, a simple plenum sheet metal cover between the intake and exhaust will help heat the intake.I saw the sheet metal baffle on the same 261 Chevy engine with the 3 Holley Webers.
You also might consider other carburetors,the Holley Webers are butt ugly looking. I like the idea of side draft carbs, saw a GMC with 3 Jaguar SU's , ran very nice,looked fantastic, but clearance can be problem.There's always the DCOE Webers if you got a large wallet or my favorite, 3 Harley S&S carbs, which can be had for about 50 bucks each at swap meets.
I do know the Holley Webers will work fine without a lot of tuning, the other carbs may require a lot of messing with jetting.
What ever you decide on ,make sure the carbs clear the fender braces or anything else in the way, good luck, let us know how you make out.
