11-12 second E/Ts would be quite something out of a Slant 6 especially without forced induction. I'd say it's possible though with the right setup and a light vehicle such as that Valiant you have there.
That thing is sweet, where can I find something like that for my 318 I'm building? THAT will be a sleeper. But your slant-6 idea is cool too though. What kind of Carb you run on a 225? (Or whatever your cubes will when you are done.)