I don't know if the mounts (rubber part) are the same. but if you ask the local parts store what there listing for a '74 250 includes, you should have your answer. There is more than one way to do the mounts, cut the '74's out and adapt them to the '51, buy some aftermarket ones, or make your own.
Too much Rube Goldberg for me on that intake idea. You would be better off selling the intake on Ebay, then take the money and buy a used 250 4bbl Offy or Clifford.
As for the radiator clearance, you have both setups, so just take a tape measure and find out for yourself what kind of room you have.
Like most any basic hopup, cam, headers and 4bbl carb is the way to go. get a copy of Leo's book on building the 250 series of engines. Plenty of info to get you started. Look in the "Club Stuff" section.
