With crummy fin-tube baseboard you get maybe 550-600 BTU/hr per running foot at an AWT of 180F, so an 8-footer is only good for about (optimistically ) 4800 BTU/hr, but more realistically closer to 4000 BTU/hr.
A 5"deep x 20" tall Arco Sunrad or Burnahm Radiant radiator puts out 170 BTU/hr per inch of running length @ 180F AWT, so even a 30-incher would put out slightly more heat than your 8' baseboard. Since you want a bit more heat into that room something between 32-40" wide is probably about right. You can build a bench over it if you like- it's about the right height. They're designed to be recessed into kneewalls or under windows, or free-standing. (They put out slightly more heat if it's free-standing than when tucked into recess, of course.)
Brand new they're pretty pricey, but these come up regularly on places like
Craigslist for cheap, since they were very popular in post-WW-II houses in the late 1940s & 50s. They clean up nicely if you want to take the trouble to sand-blast & refinish- I've seen some done with gloss automotive paints that look like a million bucks, but I usually lightly sand them by hand & use acrylic latex interior paints. (For steam heating systems use a high-temp paint.)
Several years ago I picked up a 40" and a 56" SunRad in Lyme CT ($100 for the pair- well worth the drive from Worcester MA!) for use in my own home when micro-zoning the heating system. The direct radiation aspects of them are far more comfortable than baseboard, and would make more sense than baseboard for a warming bench. The thermal mass would keep the bench from cycling hot/cold too.
One of the 27" Arco SunRads this guy is selling would be about right as a direct upgrade from your 8' baseboard at the same output, but if you want more heat hold out for a slightly longer one. They're out there, and
they show up all the time in CT/MA. Like all cast iron rads they're pretty heavy- handle with care. (Moving them from the basement where I painted them all the up to the 2nd floor by myself took some clever belayed rigging and a come-along.)