Project Black Betty
I did it with a pipe cutter. Worked OK but slow. Did the next one with a reciprocating saw. Worked every bit as well and much faster. The key is to scribe a nice line to follow and watch carefully while cutting. I used thin sheet metal wrapped around to achieve a straight line. Check with a square after cutting and make minor corrections with a hand file. The latter should also be done if the pipe cutter is used.
I used custom made bushings and a pipe to ensure everything was straight while welding so it would not really matter if the cut was perfect anyway. Details are in my build thread.