ROC pretty much nailed it. Stick with all original, or go all out. The Boss 302 engine is not all that. It had a lot of flaws.
My 67 289 Mustang completely mauled Boss 302's at the track.
If you want to go somewhat old school, then start with a set of Trick Flow Twisted Wedge heads (make sure to match the pistons) as they were the first aftermarket high performance head for the Ford small block which really worked. They are still a good head today.
Drop a 3:73 TruTrac 9" rear end in the back, hooked up through a Tremec T56 and have some serious fun. Makes a very streetable car while not giving up the ability to launch like a rocket.