Duration is RPM/displacement/head flow related.
If you want to keep the RPM at say 5500, it won't take much more that 245 or so at .050 but the issue becomes getting enough lift with that duration. You'll need 1.6 rockers most likely.
If you buy a cam with a fairly modern lobe, you don't give up much in driveability IF you can tune and IF you planned for it. I've said it before, but I run 11.08:1 with iron heads on pump gas with full timing (36 total) and my cam is 281/281 255/255 .620/.620 in a 105 in at 105. It will idle at 750 if I let it, but I keep it at about 1k to keep the lifters spinning and keep oil on the lobes.
It doesn't rattle, it doesn't chug at low RPM. My wife drives it.
But, you have to plan for it AND tune for it. If you can't do both, then go a different route and do what PRH said.
