I really love the TR4. It's such a classic looking car. Personally, I prefer inline engines in a TR, but since the firewall has already been altered and it's a solid axle car, stuff a v8 in that thing. Take a look at what's already been done (Britishv8.org), as you'd definitely need a bit of chassis stiffening. And definitely start a build thread so we can see how it goes.
