A general comment on cam choice:
First, decide at what RPM range you want the engine to operate most efficiently. That will be dictated by things like rear axle ratio, tire size, city or highway use.
Second, what maximum RPM do you need to enter the next higher gear with some reasonable torque available. For example a SM420 drops about 50% between gears, so if you want to have torque available on the 2-3 shift you might rev to 4000 RPM to come into 3rd at 2000 RPM. That would be necessary only with a fairly long duration cam -- much hotter than a 261 cam, say 220 deg. @ 0.050 lift. BTW my experience shows that such a cam gives a nice torque rise from 2600 on up but the fire goes out below about 2500.
From this analysis you can determine compression ratio requirements and carb size, both limited by stovebolt head design.
