[67FEfastback]

No one answer for this.

Power to weight ratio and value I'd have to say the 351W based variants come out on top.

Power, you are going to get more out of a 385 series engine, heavier though

If you want to remain true to the vehicle's heritage, then 260/289 block, or it's stroked cousins.
or the FE.

From a strict fan point of view, I don't care for the look of a cobra with a 385 series or 351W variant. I don't even give them a second glance at car shows.
It's like seeing a classic pontiac GTO with a 502 BBC in it. It might be fast, but who cares?

Personally I like the look of a 289 with webbers or an FE.