I chose the sierra route with the thinking of using a nice easy 1600 pinto engine then swapping it out later post iva for something with serious grunt.....I chose the serious grunt first because in hindsight I am glad I chose my prefered engine over the pinto as it would have been soul destroying to do the upgrade after iva ....my advice would be to go for the engine you want in the car with the thoughts of future tunability so the later ford twin cams are a good choice as they will give a proportionate power increase to what you spend on them.My choice of engine gave me this massive tunability but it was a hard install. If I had gone the other way I would more than likely got sick of it, just my tuppence worth
