I think this is a great idea. If I were to do it I would use the 603 engine, preferably the 350 as it makes gobs of torque. If you could find a wrecked 300 or 350 SDL and a clean SEC body you should have everything you need to do it all with factory parts. The only concern right off is whether the tall 603 would clear the sleeker hood of the SEC. I'm betting it would. Other than that you might have to do a little modification to the exhaust for the shorter body. Since both cars are 126 chassis everything else should swap between the two.