Two different blocks would be a good way to do it, but not as large as HP boilers?
Nuclear steam Mark Is top out at 1344 mb/t of steam, more with gregtech. I believe the largest boiler is 720 mB/t. You're suggesting killing nuclear output.
No, I'm suggesting a drawback to using said block... And when I was referencing the 'size', I was taking about the virtual space of the mutliblock structure that is 'Nuclear Boiler'..
Considering it would have its own 3-6 column grid, and Water/Steam internal storage, it would make the current nuclear reactor a tad bit obsolete in terms of pure MJ output (or possibly Steam EU production)
...yet it return, the current reactor could be better focused for breeding...