From what I have read, the flow heaters have to have the t-stat open to do any good. Seeing as how Kert posted something, I'd milk his brain dry for info since he is da man (well, one of several around here IMO).
You may end up finding that you like the magnet type. Sounds like you picked a decent location to install it. I am curious as to how well the magnet holds up (I have the freeze plug type) so post up your review. I'm not sure the freeze plug type is good enough on my 400 SBC because they are basically steam cooled and it seems to be lacking.