68 dart small block vs hemi

Without a doubt, the Hemi would make your car faster----AND MORE EXPENSIVE! And while your familiarity with smallblocks is a plus, there's something else to consider. The more you modify a smallblock to reach comparable hp figures as a Hemi you're also taxing its reliability more. With today's technology you could get hp numbers close to or the same with a wedge as a Hemi, but again, that's gonna cost.
If it were me, if you always wanted a Hemi, go for it. You won't be happy until you at least try it, if for no other reason than to satisfy your "itch" for one.