It matters. The only pwr steering pump in 1965 was the TRW. It has a round reservoir with 2 bolts securing it on the rear. The similar looking Federal (or "Chrysler") pump also had a round reservoir, but only 1 bolt on the rear. As far as I know, you can only get brackets to mount the Federal or Saginaw pump on the aluminum water pump (~1970+). I also think the TRW pump brackets will only mount to the cast-iron water pump (pre ~1970) and are the only brackets that do so (maybe others would w/ spacers). I know this because I switched to the alum w.p. in my 1965 273 (long story) and had to get a Federal pump. It barely clears the K-frame , using the original thick motor mounts. I later read that the Saginaw pump is best.
You can buy brackets for all 3 p.s. pumps (TRW, Federal, Saginaw) for ~$75. I did. I forgot the site, but a search will quickly find. If you buy used, insure you get the little triangle bracket that mounts to the lower block since most sellers are too clueless to provide it.
In sum, in case you are a lazy reader, if you stay cast-iron, w.p., you must use a TRW pump and brackets. That will also make your pulleys more likely to line up. I rebuilt mine w/ a kit from rockauto. Now saving it for my 65 Newport.
