ive experimented just a bit with intake length changes
^^^this on my not-a-BSA commuter bike. started and ran normally. it's a 5-inch extension that gives a total intake length of 4 inches (intake horn) + 5 inches (tube) + 2 inches (stub) + 2 inches (to valve) = 13 inches. look at the fuel taps. like gavin said, there's clearance issues, which is why i went with 5 inches.
it started and ran normally but i didn't test anything. i just wanted to see whether it would go. a twelve inch piece of tubing can be cut into 3, 4, and 5-inch sections for varying triumph intake tract length from 11, 12, and 13 inches. this is in the range of old panic's intake length predictions from his mathmatical model published in his victory series
i would have preferred to just make longer air horns but these carbs have a weird flange that requires custom mounts on the atmospheric side of the carb.
I don't recall the specifics, but I do remember that the results had to be determined experimentally. Too many variables to predict mathematically. Nowadays a computer can probably do it if it is fed enough data.
on the machine i race now, i have the carbs set as close to the head as i can get them, just to try to hold that dimension constant while i mess with exhaust and valve timing. i firmly believe that machines like ours benefit significantly from acoustic tuning of both intake and exhaust systems, but i don't think you can get anywhere quicker
with a mathematical prediction over simple cut and test.