Check out britishv8.com for ideas on engine swaps. The purists here will tell you that TR4's are becoming more rare, and are becoming more collectible each year, so it should not be hot rodded. There is some validity to this argument. Once you cut one up, it is hard to get its value back.
On the other hand, it's your car. Put your flame suit on and do whatever makes you happy. But you might try photoshop before you start hacking on an otherwise salvageable TR!
The TR4 and Spit do share a windshield frame, but that's about it.
Good luck!