Jim,
Read post 6. I think there were never brackets to mount a Saginaw or a Federal (aka "Chrysler") pump to the early cast-iron water pump (thru ~1970, lower hose on driver's side), so a TRW pump is the easy option. I know that going the other way is impossible (TRW pump on alum w.p.).
The Saginaw pump is better, so if you really want it, you might adapt the bracket. The main difference is the early w.p. has an ear sticking out for a bolt (TRW pump bracket uses), but that ear may not interfere. It is also thinner, but you could probably add spacers. The Saginaw pump sits lower, so might interfere with your K-frame. Your pulleys may not line up, but a common issue for many swaps, so browse the many photos on FABO.
I put an alum w.p. on my 1965 273, and that caused many issues. One of the biggest is that the fan then sits 1" closer to the radiator, with no room for a fan clutch. Of course, I found I couldn't use my rebuilt TRW pump.
