IMHO!
The old Ford would likely be the least desirable... probably have a heavier chassis, as in possibly too heavy. Most likely to have an undesirable Manufacturer Transaxle. Also probably the highest center of gravity.
Craftsman - dependent on age could be a good one, if it's a newer AYP Chassis, the older Craftsman mowers again get into an issue with higher center of gravity.
The Murray wide body is a fairly popular chassis among some Lawn Mower Racers. Others don't hold it in quite as high a regard because it's built a little on the light side.
The Murray 12.5 I assume is a little older machine, has a very durable stamped chassis, and offers a great deal of potential. There are a good many of them running the lawn mower circuits around the country. However, in the USLMRA they wouldn't be seen with the V-Twin as it didn't come as a stock configuration.
If you guys are going to build you might consider building to a local clubs specs however, so you can join and run with the others! Once you start it gets in your blood, and you'll want to do it all the time!