The straight pipes work like this: the length of the pipe determines the RPM range where the suction of the exiting pulse can then lower the pressure in the pipe at the head. The end cut must be square in order for this to work, however, else the suction is totally lost. The function of the megaphone shape is to follow the increasing volume of the pulse and shape it, so when it reaches the exit it exerts magimum suction at the other end of the pipe. There are many [old] texts on this topic, and the physics has not changed one iota since they were published in the 1940s. The books are incredibly hard to obtain today, drawing over $500 per copy for the few that I have been able to locate. I was fortunate enough to have one for a year in the early 1970s...
If you don't care about the performance, then cut the pipe(s) to whatever length suits. Be aware, though, that an unspecific-length pipe will cause a wide RPM range of low-pressure at the head, which will cause a draw thru the head during overlap, which in turn puts unburned fuel into the headers. This makes the exhaust side of the head and the front of the pipes hotter than normal as the burn lights with the next exhaust stroke, and can cause small backfire during decel on long hills. Installing some backpressure will reduce this heating tendency by reducing how much unburned fuel is dragged along.
You will also experience 3 RPM bands of power increase and decrease, relative to the stock pipes. I'll leave you do discover where they are, but one of them will make the bike feel very 'flat' in city traffic.
