I think I would re-enforce the 2x8 structure under the 800lb. "gorilla". That's a lot of weight, plus the water inside. A post at each end will keep the "sags" out. It will make a difference if the radiator runs parallel to the floor joist. Perpendicular, against an outside wall, not so much.
Not trying to tell you how to install this, just a suggestion. By the way,"tis a thing of beauty"!
Many of the buildings around here going back 100-200 years had the old rough cut full size lumber cut from white pine. good idea to reinforce it if the span is over 8-10'. I've seen 3-4 of them on ~14" centers crack under load. Might not bee too bad if it's really close to the wall were most of the shear load is close to the bearing point. You'll know if it feels a little bouncy!