I don't know what the Runtal radiators look like. These are cast iron radiators? If they are normally mounted flush with the wall then attaching them to the baseboards will leave an unwanted gap, won't it? If they are not supposed to be flush and a space is normally present, a little wider won't matter. There are more than one reason to close the gap between the drywall and the floor. If it can't be seen, it does not matter much how you close it. Quarter round with some caulk sealant should work fine.

I don't entirely understand what mean by "bleed the baseboards..." If you have radiators, a baseboard is superfluous. Bleeding them in in the vertical direction will look too busy for me. On the other hand, you won't have radiators along the entire length of the walls so you will want a baseboard where there is no radiator and you want the transition to look pleasing. That is a judgement call and depends on what else you are doing with the room. In general, however, I'd probably like to line up the top of the rads with something that resembles them very much even if it does not stand as far proud of the wall. That is just my preference and it assumes a boxy radiator with a pretty modern look. Matching it might mean some custom millwork.