There are 2 slant six's. The bigger one, I think a 225 or 226 ci, and a smaller one that is in the 177-ish range. Both of them are good engines, the smaller one doesn't make a lot of power, but is damn reliable, you can literally run cooking oil in the sump if you wanted or had to. (Don't ask how I know) the 225 makes decent horsepower, but it is designed more for torque, which is good. Both engines have plenty of parts available and are fairly cheap and easy to work on, even in that cramped engine bay.
BTW, I've personally seen a 177ci with a pushbutton auto running sub 10 second 1/4's driven by a guy in his 70s. They definitely have potential.