Well for the best streetablity I'd say a stock motor is great.
Also 300+ hp is easily obtained with a stock motor as well. I mylself did 8,000 miles on a completly stock motor last year. Only thing done was a spacer plate under the block.
I'd just turbo what you've got, maybe throw in some pistons. And when and if you get bored with that, then do a 1390 later.
But really it's all in what you want to do. If you want to make big hp, do pistons, rods, springs, and a new cam chain