I think setting up the engine is the hardest, most challenging, but most important aspect of building a car. The suspension, wheels/tires, and chassis reinforcing is just as important, but a lot easier, so I say do the engine first...get a foundation and get it running, then take care of the less intensive items and get it set up how you want.
My first 1200 I did converted's approach, it handled AWESOME but was a slouch with the A12. It was great for a beginner, but I quickly grew out of that and wanted more power. For trackwork it would've been perfect, but for street driving I could only get guys if we were on a curvy road which didn't happen very often. I guess it just depends on what you want to do with it.
Hey Craig, is 300 rwhp beyond the chassis competency of the 1200? I think so

but I wouldn't have it any other way. All the Japanese tuners/racer are calling it "Danger Car" LOL