Sorry guys I'm not going to say anything exotic so feel free to skip reading this post!
I always liked the look of the RB motors, so I'd probably build a 440 or 426 stroker motor and saddle it with a blower or a turbo. Blower seems more appropriate. I'd dump it into a black GTX or charger.
I've always had a thing for Flatheads, I would probably mess with one if I had the time/space/money.
#1 on the list would probably be a stroked 426 though. I like the valve covers, the history, the legend/awe factor etc. Seems like they present a challenge too, because they seem to require a new learning curve in terms of tuning. I think I'd enjoy the challenge and reward.