2.125" would be ok for decent performance with that CI assuming that you are not revving over 7000 RPM by much. 2.25" would be better if you are a NO2 guy. Either size you choose to use, be sure that you make all pipes 30" long measured from the flange, no longer, unless your engine resides in the lower rpm range like 6000 or less then 32" would be better. The smaller pipe you use meaning the 2.125", the more effective the lower end torque will be and higher the vacuum signal will be at the carb to make tuning very easy. Do not make the pipes shorter than 27" unless you rev over 7600 rpm. Good luck on your fab, headers can be a very rewarding job to do and not real complicated either. Just be sure that you do NOT drop your weld in the pipe. The inside of a pipe joint should not look like it has been welded on the inside. This will restrict air flow and make the header act like a smaller diameter. Keep the weld on the outside of the pipe only.
Bruce