Here's the M/C you're looking for.
Note the casting number on the bottom 2225541. According to the link to the article Steve posted in #3, this is what you need. Be aware that the original version of this article had a typo, with the incorrect number shown.
Also note the flat machined front with no "snout" like the one pictured in post #4.
These are pretty expensive, but could be your best bet if you want to have the correct appearing one. Quite a while ago, the best price I could find for a rebuildable core was close to $200.
NOS Brake Master Cylinder 67 68 69 70 Dodge Dart Custom Swinger w/ Front Disc | eBay
