My update on this for the 750:
I tried flashing 50 and 57 megs for root partition and the installer didn't re-partiton.. it seem to just hang at the Please Wait screen.
However, after talking to some people on the Cacko channel, they suggested I try using 121 (megs) to force the installer to create one whole root partition.
This worked for me, and was what I wanted anyways, so now I'm flashing in the new rom image.
Update: Ignore what I wrote up there. Don't use 121 for a 750. I don't know what value to use, but 121 won't work. I've gone back to 50 and it works.