That'll be an aluminum 225. At one time I had five of them (now down to two). I used to like to call up the machine shop, tell them I was bringing in a Dodge 225 slant-6, and then walk in through their front door carrying the block in my two hands and say "Here's that Dodge 225 I called about; where do you want me to put it?". :toothy10:
Price is reasonable if it's a decent basis to work from, but before you go to the trouble and expense, pull the cylinder head and take a look at the condition of the cylinders and the inside of the block (it's an open-deck design; you'll be able to see right down into the whole of the block). If there's serious corrosion -- and there might well be -- the block will require expensive refurbishing to be usable. That doesn't necessarily mean you shouldn't get it, just be aware of what you're getting yourself into.
Should you build it for your '69 Dart? That depends on what kind of buildup you have in mind. These engines live a long and happy life with stock compression, but they
do not happily handle any kind of a substantial increase in compression; the special aluminum-engine-only head gasket is 1961 technology and there's (still) no updated one. Even if there were, the freestanding cylinder barrels are highly prone to shift-and-drift. Also, if you want to run it in front of an automatic trans in your '69, you'd either need a '66-'67 trans or a custom torque converter w/small '67-down nose on the front and '68-up splines on the back, or a '68-up crankshaft, or machine the '62 crank's counterbore 1/4" larger in diameter to accept the '68-up large-nose torque converter.
Lots of good threads over on slantsix.org about special builds on aluminum 225s.
This one and
this one are especially good, with tons of pics and descriptive text. There are others, too
