Shotgun minimum overall legnth in the USA is 22", minimum barrel legnth 18" which would in no way be approached with an 18" barrel Saiga, so you are OK- even so, OAL is measured on folding/collapsable stocked weapons with the stocks extended.
The Saiga 19" versions are 19" because that is the Russian barrel legnth standard for ordinary folk to own them.
Unless you are remarkably talented, hacksawing a barrel without first scoring it with a pipe cutter or similarly orientede tool is the fastest way to make your barrel look like ass.