From what you are describing, I think you aren't going to get away easy on this one... If you have access from below, you could spin out the nipples, and recut & thread new ones 3/4 longer- I assume you are dealing with unions... This would be the correct way to do it.
Or, alternatively, perhaps you could bore out a 3/4 deep plug with a hole saw for the legs to drop into? If you keep them tight it probably wouldn't look too bad... you could skirt the bottom of the radiators with a nice oak 1x4 box stained to match?
I don't know, it's a head scratcher. This is something that always has to be taken into account before you layer or change an elevation.