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Dedicated basement zone, what radiator type to prevent short cycling?

I'm finishing about 300 sq. ft. of my basement, and have re-configured my boiler to have a second, dedicated zone. I know that finned tube baseboard is not an option for heating the space, because it would heat and cool too quickly and potentially cause the boiler to short cycle. Ideally I would install a cast iron radiator, since it has a high thermal mass and water volume, and it would match the ones in the rest of my house, but I'm having trouble finding one of the right size/type/condition locally.

Would the Runtal baseboards be an option to consider/ I know they don't really hold more water than slantfin baseboard, but wasn't sure if they have more thermal mass. Any thoughts are appreciated.

I understand another option would be a configure the zone relay so that the basement circulator is controlled by the thermostat, but it will only turn the boiler on when the upstairs zone is also running. This would probably work fine for our lifestyle, but I'd ideally have two dedicated zones.

Boiler is a Burnham Alpine 80, which can do 16-80 MBH. This zone would need abotu 12,000 btu/hr if above grade, so I'm thinking 8000 btu/hr should be safe since it's a basement.

Any insight on radiator type or other solutions are appreciate, thanks!