I did a side by side comparison of the big block water pump housings and here's what I found.

The older restrictive 440 Source housing has seriously restrictive small lower passages where it turns going to the engine block.
The passage size is roughly 3/8th of an inch, roughly 1/2 the size of a factory cast iron housing.
You can barely get your pinkie finger into the lower water passages through the center hole where the water pump bolts on.
Waaay to small.

No amount of grinding with a carbide bit would ever be able to get the passages to the size of a factory housing. You would grind through the wall long before it was larger enough.

On a mild stock type engine they might work okay, but on a stroker motor like mine it would never let it flow enough.

The factory cast iron housing passages are large and roughly 5/8th of an inch. You can easily put any finger into the passage from the front with room to spare.

The new 440 Source housing passages are roughly 9/16th of an inch in size. A tiny bit smaller than the cast iron housing but waaay larger that the earlier versions.

I used a carbide bit to open it up a bit and smooth the corners of the passages.
By he time I was done it should flow the same as a factory housing.

What may be kind of humorous is I asked 440 Source about the earlier restrictive housings and I was told that only one passage was too small and restrictive. Not two passages like I was telling them.

Both lower passages on the early version are about 1/2 the size of the new version from 440 Source.