I'm going to side with Dana on this one. 16' is definitely way too short, since the math says that 50' itself is already too short.
Unless, of course, your goal is to fry your boiler and get the insurance company to pay for a new one. YMMV.
If we're settled that extreme short cycling is something to avoid, I see three viable options:
1. Run the basement baseboards off the same zone as the 1st floor. I'd put it in series, after the 1st floor, so the basement "sees" slightly cooler water. This will still almost certainly overheat the basement, but warm air rises and you'll get most of that back. Could make the basement uncomfortable, though. Alternately, you could plumb it in parallel and put a gate valve onto it to restrict flow, thus manually tweaking the balance.
2. Put 50' or so of baseboards in so that the basement zone is more like the others. Ideally, you should probably downsize the boiler next time you replace it, or maybe run an indirect water heater off it as well to make use of its excess capacity. The more perfectly the boiler is matched to the radiant (and indirect) load, the more time it will spend in its steady-state efficiency zone. The idea here is to make it run in short normal cycles, rather than short-cycling. The difference is in whether the room thermostat or the boiler's internal thermostat is making the shutoff decision.
3. Hang it all and just put independent electric baseboards down there.
For what it's worth, I don't see an overly warm basement as an entirely bad thing. It reduces the heat load above, and buys time when the power goes out before your pipes are at risk of freezing. Just so it's not so hot you can't go down there.