All the time and money spent of turning other V6 engines into ST200 spec is wasted IMHO, why not just get an ST200, the car is a lot more than just the engine.
Stu
Because
A) ST200s are ludicrously expensive for what they are and it's cheaper to actually get a standard car up to st200 spec by a fair way than to buy one outright, plus you can do it over time rather than having to fork out in one big chunk
B) You say the ST200 is more than just the engine...you're right, it's the suspension too but even that isnt as good as a proper aftermarket kit like the Koni Yellow adjustables....so why spend the extra on the st200 when you've got to mod that anyway? Example: Almost every contour runner in america is using the koni yellow adjustables, yet their cars come with the exact same set up as the st200 iirc.
c) Modding is fun
Lower inlet manifold, upper inlet manifold and throttle body: £150
Twin Exit Powerflow exhaust (you'd do that to an st200 anyway, still a restrictive system) £300 or so, or in my case £50 used
ST200 Heads and cams: £150
Koni adjustable suspension kit: £425
So, as you can see.....that'd get your car to better than st200 spec (bar wishbones and anti roll bar, could get an even better ARB for £80 though) for under £800

Oh, you also get cruise control, heated electric leather etc whcih the st200 doesnt have.
Tom.