It's highly unlikely that Casco will send the cast impeller to you, but they will sell the latest generation pump/w cast impeller. Expect to pay about $30 for shipping. You can get the same pump from Concours, and they are having a 10% off Labor Day Sale through the 9th. 

THANK YOU blacktie!
I misread their catalog blurb. It DOES say the upgrade is ‘2nd gen.’. I only assumed they would naturally use their latest impeller design (go figure).  Also, they charged me $60 (inflation?) to supposedly reserve an impeller (catalog stated ‘while supplies last’, so I also ‘assumed’ a 3rd gen. production/delivery problem. Rather deceptive on their part, creeping senility on my part).
I will likely apply the $60 toward a new pump, or maybe they will just send me a 3rd gen. impeller, if they have them stocked separately, and it will merely press-on (?) whatever pump shaft I want (I have an alum. pump on my spare motor).

6 VOLTS/POS. GRD. NW INDIANA

If I recall correctly, Casco's $50 upgrade consists of adding the riveted vanes to your pump. They will not replace yours with the 1-piece cast vanes (I wish they did). I have one of their riveted pumps; although it's still intact (little use), I don't intend to use it when my engine is rebuilt. 

Phil,
Which gen. CASCO pump do you have? While speaking with Chris (Paragon) re: his improved spacer design, the subject of 1st gen. Casco riveted pump blade failures came up.  With my coolant drained & fan removed, I noticed a subtle tinkling sound when spinning the pump shaft (loose rivets?). According to Chris, when those blades finally do come off, they sometimes make a hole in the spacer, and result in sudden coolant loss. Current 3rd. gen. pumps have a solid impeller with deeper vanes, and they can upgrade your pump ($50), something I plan to do though I have to wonder how much life is left in the pump bearings after running their (nose-heavy) clutch fan setup for over 25K miles? (no discernible side play, but impeller/shaft DOES spin quite easily).

6 VOLTS/POS. GRD. NW INDIANA

How many times did motorists go out of their way to tell you that your hood was open?

It's just a simple nylon strap with a plastic buckle, like what is on a life jacket. You could use something else similar, certainly safer. If you don't use the strap, at approx. 70 mph the hood will only rise up an inch or 2 (DON'T EVEN TRY THIS AS IT'S DANGEROUS AS HELL!) BUT, I used to do it when I was about 18 yrs old all the time, so that is my research, haha. Everyone knows you are invincible at that time in your life! So, what I learned is one could control or curb the rise of the hood with a cinch strap or two (better) in case of any kind of gust or crosswind. Like I said I'm not recommending it for you. But it does help.  Lovering the trailing edge of the hood would accomplish the same thing but obviously an irreversible solution. I strapped it from the hood release bar to the springs on the hood pins. I haven't done this in a while since the need hasn't arisen.

       A  L  A  N   F  R  A  K  E  S   ~  Tulsa, OK    

I would like to see a photo of the cinch strap.

And then there is mine which is a 7-blade flex fan. not sure where I got it but it has been working well for at least 40 years. Lots of details to consider cooling off the little bird. The biggest difference in all my mods and attempts was not on purpose, It's when I added the FPR headers. much quicker cool down, less heat soak.  Also, something that is always overlooked and dismissed is the depth of the fan blades, the fit, and SEAL of the fan shroud, AND the depth at which the fan enters the shroud. lots to be experimented with and no sure-fire solution. When I run down the hiway on a trip during summer I open the hood and use a cinch strap to keep it secure. The heat dumps out pretty well this way.

       A  L  A  N   F  R  A  K  E  S   ~  Tulsa, OK    

At least your fan blades were not installed backwards like mine was on my '57 Tbird when I acquired it.  It is a 6-blade model, but somebody got confused. 
Ralph

When I first obtained my ‘55 Bird, I had the mechanic who prepped it install a 4-blade fan (the orig. chromed 3-blade part had been replaced with a black one long ago though).  Years later I had it powder coated silver (easier to clean). As it isn’t correct for a ‘55, and I won’t give up my 6-blade/clutch fan since my car is a ‘driver’, the O.P might want it?

6 VOLTS/POS. GRD. NW INDIANA